Main

We investigate the effects of strong magnetic fields on the equation of state (EoS) of neutron star matter and the resulting implications for tidal deformability measurements in binary neutron star (BNS) mergers. A critical issue with previous magnetized neutron star studies is the treatment of magnetic field anisotropy in the Tolman-Oppenheimer-Volkoff (TOV) equations. To address this fundamental problem, we employ the chaotic magnetic field approximation, which allows for a self-consistent treatment of magnetic pressure while maintaining isotropy. Using a relativistic mean field approach with properly implemented magnetic field corrections, we compute mass-radius relations and tidal deformability parameters for neutron stars with magnetic field strengths ranging from $10^{15}$ to $10^{16}$ G. Our systematic study reveals that magnetic fields induce increases in both stellar radii (0.8–2.3\%) and tidal deformabilities (4.2–18.1\%) compared to field-free cases, with effects scaling approximately as $B^{1/2}$. These modifications, while modest, are potentially detectable with current and next-generation gravitational wave detectors. For a canonical $1.4\,M_\odot$ neutron star, the tidal deformability increases from $\Lambda_{1.4} = 678 \times 10^6$ in the absence of magnetic fields to $\Lambda_{1.4} = 803 \times 10^6$ for $B = 10^{16}$ G. We demonstrate that magnetic field effects must be considered when constraining the neutron star equation of state using gravitational wave observations, particularly for populations including highly magnetized neutron stars. Our results suggest that the current GW170817 constraint on tidal deformability may require systematic corrections when accounting for magnetic field effects. We provide scaling relations for magnetic field corrections and discuss the implications for population studies of neutron star mergers with next-generation detectors.

Multiply-imaged supernovae (SNe) provide a novel means of constraining the Hubble constant ($H_0$). Such measurements require a combination of precise models of the lensing mass distribution and an accurate estimate of the relative time delays between arrival of the multiple images. Only two multiply-imaged SNe, Refsdal and H0pe, have enabled measurements of $H_0$ thus far. Here we detail the third such measurement for SN Encore, a $z=1.95$ SNIa discovered in JWST/NIRCam imaging. We measure the time delay, perform simulations of additional microlensing and millilensing systematics, and combine with the mass models of Suyu et al. in a double-blind analysis to obtain our $H_0$ constraint. Our final time-delay measurement is $\Delta t_{1b,1a}=-39.8_{-3.3}^{+3.9}$ days, which is combined with seven lens models weighted by the likelihood of the observed multiple image positions for a result of $H_0=66.9_{-8.1}^{+11.2} \rm{km} \rm{s}^{-1}\rm{Mpc}^{-1}$. The uncertainty on this measurement could be improved significantly if template imaging is obtained. Remarkably, a sibling to SN Encore (SN "Requiem") was discovered in the same host galaxy, making the MACS J0138.0-2155 cluster the first system known to produce more than one observed multiply-imaged SN. SN Requiem has a fourth image that is expected to appear within a few years, providing an unprecedented decade-long baseline for time-delay cosmography and an opportunity for a high-precision joint estimate of $H_0$.

For nearly a decade, observations have shown that many older Sun-like stars spin faster than predicted, a phenomenon known as weakened magnetic braking (WMB). The leading hypothesis for WMB is a weakening of the large-scale dipole field, which leads to a less efficient angular momentum loss. To test this hypothesis on a star known to be in the WMB regime, we present the first Zeeman Doppler Imaging (ZDI) map of the Sun-like star $\tau$Ceti, reconstructed using spectropolarimetric data from the Canada-France-Hawai'i Telescope (CFHT). Our ZDI analysis reveals a remarkably simple, stable and weak ($\langle B\rangle =0.17 \mathrm{G}$) magnetic field, characterized by a predominantly dipolar ($\sim92\%$ magnetic energy contained in $l=1$ modes), and highly axisymmetric ($\sim88\%$ magnetic energy contained in $m<l/2$ modes) morphology. We infer a dipole field strength of $B_{\mathrm{dip}}=0.31 \mathrm{G}$, nearly an order of magnitude weaker than standard braking model predictions, providing direct confirmation of the weakened large-scale dipole predicted by the WMB hypothesis. This work establishes a new benchmark for ZDI, demonstrating that even extremely quiet stars in the WMB regime are accessible to this technique.

Exoplanet demographics increasingly reveal that planetary properties depend not only on local irradiation and composition but also on the wider system architecture. We analyse a sample of Neptune-sized short-period planets with well-measured masses and radii, identifying those whose host stars harbour at least one confirmed outer-giant (OG) companion. On the mass-radius (M-R) plane, the two populations diverge modestly: inner planets in OG systems cluster at systematically larger radii than their counterparts in no-giant (NG) systems, a result that remains suggestive after controlling for planet and stellar properties. Bayesian modelling quantifies the offset, revealing an average radius enhancement of $17 \pm 4 \%$ for inner planets in OG systems relative to NG systems at fixed mass. Alternative cuts, including the use of a homogeneous set of parameters, confirm the robustness of the signal, though the result still relies on small-number statistics. Possible mechanisms for the observed inflation include boosted envelope accretion, reduced atmospheric loss, or volatile enrichment by giant-planet stirring. If upheld, this empirical link between outer giants and inflated inner-planet radii offers a new constraint on coupled formation and evolution in planetary systems.

We present a model-independent null test of the late-time cosmological response to a reduced sound horizon, as typically required by early-universe solutions to the Hubble tension. In this approach, we phenomenologically impose a shorter sound horizon without modeling early-universe physics to isolate its impact on late-time dark energy inference. Using baryon acoustic oscillations (BAO), supernovae (SN), big bang nucleosynthesis (BBN), and local $H_0$ data, while explicitly avoiding CMB anisotropies, we examine how this calibration shift propagates into constraints on the dark energy equation of state. We find that lowering $r_d$ systematically drives the $w_0$-$w_a$ posterior toward less dynamical, quintessence-like behavior, bringing it closer to $\Lambda$CDM. This result underscores that some of the apparent evidence for evolving or phantom-like dark energy may reflect early-universe assumptions rather than genuine late-time dynamics. More broadly, our analysis highlights the importance of carefully disentangling calibration effects from physical evolution in interpreting forthcoming results from DESI and future surveys.

Given the vast number of stars that exist within binary systems, it remains important to explore the effect of binary star environments on the formation and evolution of exoplanetary systems. Nearby binaries provide opportunities to characterize their properties and orbits through a combination of radial velocities, astrometry, and direct imaging. Eta Cassiopeiae is a bright, well-known binary system for which recent observations have provided greatly improved stellar masses and orbital parameters. We present additional radial velocity data that are used to perform an injection-recovery analysis for potential planetary signatures. We further provide a detailed dynamical study that explores the viability of planetary orbits throughout the system. Our combined analysis shows that giant planets are significantly ruled out for the system, and indeed no planetary orbits are viable beyond $\sim$8 AU of the primary star. However, terrestrial planets may yet exist within the Habitable Zone where orbits can remain long-term stable. We discuss the implications of these results, highlighting the effect of wide binary companions on giant planet formation, and the consequences for occurrence rates and planetary habitability.

We develop a two-scalar field quintom model, which utilises both a quintessence-like and a phantom-like scalar field, enabling a smooth and stable transition across the $w=-1$ phantom divide as hinted by recent measurements of Baryonic Acoustic Oscillations (BAO) by the Dark Energy Spectroscopic Instrument (DESI) Data Release 2. We explore a range of initial conditions and potential configurations that facilitate such a phantom-to-quintessence-like crossing, and find that this can be naturally realised with hill-top or cliff-face potentials bound from above. We study how varying these conditions affects the dynamics of the system, calculate the background observables and compare them with DESI, CMB, and Type Ia supernova data, identifying a viable parameter space for our model. In particular, we find that a potential featuring a hyperbolic tangent form can successfully reproduce the desired phantom crossing, although such models can suffer from fine-tuning effects. Finally, we discuss prospects for distinguishing such models with upcoming state-of-the-art cosmological observations.

SN 2024aecx is a nearby ($\sim$11 Mpc) Type IIb SN discovered within $\sim$1 d after explosion. In this paper we report high-cadence photometric and spectroscopic follow-up observations, conducted from as early as 0.27 d post discovery out to the nebular phase at 158.4 d. We analyze the environment of SN 2024aecx and derive a new distance, metallicity and host extinction. The light curve exhibits a hot and luminous shock-cooling peak at the first few days, followed by a main peak with very rapid post-maximum decline. The earliest spectra are blue and featureless, while from 2.3 d after discovery prominent P-Cygni profiles emerge. At nebular phase, the emission lines exhibit asymmetric and double-peaked profiles, indicating asphericity and/or early dust formation in the ejecta. We simulated the progenitor and explosion using a two-component model of shock cooling and radioactive $^{56}$Ni heating; our model favors an extended, low-mass H-rich envelope with$ M_{\mathrm{e}} = 0.08^{+0.02}_{-0.03}\, M_{\odot} $ and a low ejecta mass of $ M_{\mathrm{ej}} = 2.65^{+1.21}_{-0.73} \, M_{\odot}. $The comprehensive monitoring of SN 2024aecx, coupled with the detailed characterization of its local environment, establishes it as a benchmark event for probing the progenitors and explosion mechanisms of Type IIb SNe.

The nature of dark energy remains one of the most important unanswered problems in physics. Here we use gamma-ray spectra from the Type Ia supernova 1991T to constrain the recent evolution of a dynamical pseudoscalar quintessence-like field $Q(t)$. We found that the 1991T gamma rays emitted by the $^{56}\text{Fe}$ nuclei observed by COMPTEL aboard the Compton Gamma Ray Observatory were slightly shifted to lower energies with respect to terrestrial values, with the average fractional energy shift of both the first and second excited states found to be $\delta E/E = -0.006\pm0.008$ including statistical and systematic errors. Assuming that this energy shift is caused by a dynamical QCD axion-like pseudoscalar field $Q(t)$, we find that observed energy deviations are consistent with a fractional rate of change of the pion mass given by $\delta \dot{m_{\pi}}/m_{\pi}=-(6\pm9)\times10^{-11}\text{ yr}^{-1}$. The observed energy deviation was also used to determine the rate of change of the quintessence-like field ($\dot{Q}_0$) for tracking models: $\dot{Q}_{0,max} = (3\pm 4)\times10^7 \text{ GeV/yr}$. Our findings are consistent with the cosmological constant ($\dot{Q}_0 =0$). Furthermore, we have demonstrated how nuclear spectra produced by astrophysical events can be used to inform the nature and behavior of dark energy.

Hydrogen-rich supernovae (SNe) span a range of hydrogen envelope masses at core collapse, producing diverse light curves from extended plateaus in Type II SNe to double-peaked Type IIb SNe. Recent hydrodynamic modeling predicts a continuous sequence of light-curve morphologies as hydrogen is removed, with short plateau SNe (plateau durations  50–70 days) emerging as a transitional class. However, the observational boundary between IIb and short-plateau remains poorly defined, and thus far unobserved. We report on extensive photometric and spectroscopic follow-up of SN 2023wdd and SN 2022acrv, candidate transitional events on the low-mass end of the short-plateau class. Both exhibit weak, double-peaked light curves which we interpret as exceptionally short plateaus (10–20 days), and hybrid spectral features: persistent H$\alpha$ absorption with He I contamination, but without the helium dominance characteristic of IIb SNe. Using analytic shock-cooling models and numerical light curve fitting, we estimate hydrogen-rich envelope masses of  0.6–0.8 $M_\odot$ – significantly larger than canonical IIb values ($\lesssim0.1\,M_\odot$) but consistent with the ${\sim}0.9\,M_\odot$ threshold predicted for short-plateau behavior. Although the progenitor radii inferred from analytic and numerical methods differ by factors of 2–5, envelope mass estimates are consistent across approaches. Comparisons to well-studied IIb (SN 2016gkg, SN 2022hnt), short-plateau (SN 2023ufx, SN 2006ai, SN 2016egz, SN 2006Y), and II SNe (SN 2023ixf, SN 2013ej) suggest a monotonic relationship between hydrogen envelope mass and plateau length consistent with analytic and numerical expectations. These findings provide additional evidence for a continuous distribution of envelope stripping in hydrogen-rich core-collapse progenitors and place SN 2023wdd and SN 2022acrv along the IIb/short-plateau boundary.

Hot Jupiters and their atmospheres are prime targets for transmission spectroscopy due to their extended atmospheres and the corresponding large signal-to-noise, providing the best possible constraints for the atmospheric carbon-to-oxygen (C/O) ratio and metallicity of exoplanets. Within BOWIE-ALIGN, we aim to compare JWST spectra of a sample of orbitally aligned and misaligned hot Jupiters orbiting F-type stars to probe the link between hot Jupiter atmospheres and planet formation history. Here, we present a near-infrared transmission spectrum of the aligned planet KELT-7b using one transit observed with JWST NIRSpec/G395H. We find weak features, only tentative evidence for H$_2$O and CO$_2$ in the atmosphere of KELT-7b. This poses a challenge to constrain the atmospheric properties of KELT-7b and two possible scenarios emerge from equilibrium chemistry and free chemistry retrievals: a high-altitude cloud deck muting all features or an extremely low metallicity atmosphere, respectively. The retrieved C/O ratios from our data reductions range from $0.43 - 0.74$, while the atmospheric metallicity is suggested to be solar to super-solar ($1-16 \times$ solar). Although these wide constraints prevent detailed conclusions about KELT-7b's formation history, a solar-to-super-solar metallicity would imply the accretion of solid material during its formation, which is valuable information for the survey's wider goals of understanding the relative importance of gaseous to solid accretion.

The almost simultaneous detection of GRB170817A and GW170817 ushered in nearly a decade of interest in binary neutron star mergers and their multi-messenger signals, resulting in a greater understanding of the processes that produce short-duration gamma-ray bursts and gravitational waves. However, open questions remain regarding the emission mechanism of these bursts. In this work we present results from the first study of an electromagnetic signal produced from a realistic treatment of a binary neutron star merger, both for on-axis and off-axis observations. We accomplish this by using the PLUTO hydrodynamical code to inject a relativistic jet into the ejecta of a realistic binary neutron star merger, which was itself obtained from the simulation of a 3D BNS merger. Then, we model the prompt photospheric emission that would emerge from this jet using the MCRaT radiative transfer code. We find that the resulting photon spectra can peak around  1 MeV for on-axis emission and falls off noticeably for off-axis observations. We also find distinctly non-thermal low and high-energy tails in multiple observations, ranging from shallow to mid-off axis observations. Our on-axis results are consistent with the Amati Correlation for short bursts, with some strain evident at higher observing angles. Finally, we find that the radiative efficiency is much lower than seen in previous studies of the photospheric emission of long-duration gamma-ray bursts.

Asteroseismology, the study of stellar oscillations, and stellar modeling both offer profound insights into the fundamental properties and evolution of stars. With pySYD, a new open-source Python package, we were able to constrain the asteroseismic global parameters, $\nu_{max}$ and $\Delta\nu$, for 82 solar-like oscillating subgiant and lower red giant stars, filling in the region between the Kepler dwarfs and giants. Using asteroseismic scaling relations, we were able to compute seismic masses, radii, and surface gravities for our entire sample with average errors of 0.21 $M_{\bigodot}$, 0.27 $R_{\bigodot}$, and 0.06 dex respectively. Using 4 stellar modeling grids we determine and compare stellar ages for our sample. We find that our age distribution from stellar modeling is consistent with other local star samples. We find small consistent offsets from model predictions across our regime, but offsets were worse at higher gravities (log(g) $\geq$ 3.5 dex), suggesting the need for better calibration. Finally, we discuss our sample in the context of galactic archaeology and show how ages like these could be used to identify and study binary system evolution and galactic evolution in the future. All in all, we show that asteroseismology can be successfully performed with TESS data and can continue to make an impact on our understanding of stellar physics and galactic archaeology.

Stellar rotation is a fundamental parameter governing a star's magnetic activity and evolution. The Transiting Exoplanet Survey Satellite (TESS) provides high-precision photometric data ideal for measuring rotation periods via brightness modulations from starspots. This paper presents a detailed analysis of the star TIC 445493624 using 2-minute cadence data from TESS Sector 58. We process the light curve using a custom pipeline to perform outlier removal, binning, and Savitzky-Golay detrending to isolate the stellar variability. A Lomb-Scargle periodogram of the cleaned data reveals a single, dominant periodic signal at 3.638 days with a power of 0.43, corresponding to a negligible false-alarm probability. The phase-folded light curve at this period is highly coherent and exhibits a stable, non-sinusoidal morphology indicative of large-scale magnetic features or spot groups.

We review the state of the art in the detection of extreme high-energy neutrinos, focusing on the IceCube and KM3NeT neutrino telescopes. IceCube, operating deep in Antarctic ice, and KM3NeT, a new array in the Mediterranean Sea, employ distinct designs to capture Cherenkov light from neutrino interactions. We examine their detector architectures, readout and reconstruction performance for PeV-scale and higher-energy neutrinos. Recent candidate events above 5 PeV are highlighted. These include a  120 PeV muon track observed by KM3NeT in 2023, and IceCube's highest-energy detections, which comprise several-PeV showers and tracks. We outline current approaches to neutrino energy reconstruction and explore scenarios that might explain the apparent differences in observed event characteristics. Finally, we summarize future prospects for extreme-energy neutrino observations and their implications for astrophysical source populations and cosmogenic neutrinos.

Understanding the dynamical structure of cislunar space beyond geosynchronous orbit is critical for both lunar exploration and for high-Earth-orbiting trajectories. In this study, we investigate the role of mean-motion resonances and their associated heteroclinic connections in enabling natural semi-major axis transport in the Earth-Moon system. Working within the planar circular restricted three-body problem, we compute and analyze families of periodic orbits associated with the interior 4:1, 3:1, and 2:1 lunar resonances. These families exhibit a rich bifurcation structure, including transitions between prograde and retrograde branches and connections through collision orbits. We construct stable and unstable manifolds of the unstable resonant orbits using a perigee-based Poincaré map, and identify heteroclinic connections - both between resonant orbits and with lunar $L_1$ libration-point orbits - across a range of Jacobi constant values. Using a new generalized distance metric to quantify the closeness between trajectories, we establish operational times-of-flight for such heteroclinic-type orbit-to-orbit transfers. These connections reveal ballistic, zero-$\Delta v$ pathways that achieve major orbit changes within reasonable times-of-flight, thus defining a network of accessible semi-major axes. Our results provide a new dynamical framework for long-term spacecraft evolution and cislunar mission design, particularly in regimes where lunar gravity strongly perturbs high Earth orbits.

During the formation of rocky planets, the surface environments of growing protoplanets were dramatically different from those of present-day planets. The release of gravitational energy during accretion would have maintained a molten surface layer, forming a magma ocean. Simultaneously, sufficiently massive protoplanets could acquire hydrogen-rich proto-atmospheres by capturing gas from the protoplanetary disk. Chemical equilibration among the atmosphere, magma ocean, and iron core plays a key role in determining the planet's interior composition. In this study, we investigate terrestrial planet formation under such primitive surface conditions. We conduct N-body simulations to model the collisional growth from protoplanets to planets, coupled with chemical equilibrium calculations at each giant impact event, where surface melting occurs. Our results show that planetary growth proceeds through a series of giant impacts, and the timing of these impacts relative to the dissipation of disk gas significantly influences the volatile budget. In particular, initial impacts, occurring while nebular gas is still present, can lead to excess hydrogen incorporation into the protoplanet's core. Subsequent impacts with hydrogen-poor bodies, after gas dispersal, can dilute this hydrogen content. This process allows for the formation of a planet with a hydrogen inventory consistent with Earth's current core. Our findings suggest that late giant impacts, occurring after the depletion of nebular gas, provide a viable mechanism for producing Earth-like interior compositions near 1 AU.

We present observations of comet 67P/Churyumov-Gerasimenko during its 2021/22 apparition, aiming to investigate its dust and gas environment and compare the results with those obtained in 2015/16 using the same telescope. Quasi-simultaneous photometric, spectroscopic, and polarimetric observations were carried out at the 6-m BTA SAO telescope. The comet was observed on 6 October 2021, 31 days before perihelion, with \textit{g}-SDSS and \textit{r}-SDSS filters, and on 6 February 2022, 96 days after perihelion, using narrowband cometary filters: BC ($\lambda4450/62$ Å), RC ($\lambda6839/96$ Å), and CN ($\lambda3870/58$ Å). These were complemented by images from the 2-m Liverpool Telescope (La Palma). On 6 October 2021, a sunward jet and long dust tail were detected. By 6 February 2022, the dust coma morphology had changed noticeably, revealing a bright sunward neckline structure superimposed on the projected dust tail, along with two jets at position angles of 133$^{\circ}$ and 193$^{\circ}$. Spectra showed strong CN emission, with relatively weak C$_2$, C$_3$ and NH$_2$ emissions. The dust production rate $Af\rho$ did not exceed 200 cm (uncorrected for phase angle) in both epochs. An unusual CN coma morphology was observed, with evidence of an additional CN source associated with dust jets. Geometric modeling of the jets' dynamics indicated an active area at latitude $-70^{\circ} \pm 4^{\circ}$ with a jet opening angle of $20^{\circ} \pm 6^{\circ}$ on 6 October 2021, and two active areas at latitudes $-58^{\circ} \pm 5^{\circ}$ and $-53^{\circ} \pm 10^{\circ}$, separated by longitude $150^{\circ} \pm 20^{\circ}$, producing the observed jets on 6 February 2022. The average particle velocity in the jets was about $0.32 \pm 0.04$ km s$^{-1}$.

We run numerical simulations to study high-power wind accretion in a massive binary system during a high mass loss event. The system consists of an evolved primary star with a zero age main sequence mass of $ M_{1} = \rm 100~M_{\odot}$ and a hot secondary star with a mass ranging from $ M_{2} = \rm 30-80~M_{\odot}$, orbiting in a circular orbits with periods between 455 and 1155 days. We initiate a weak eruption event with mass loss at a rate of $10^{-3}~\rm {M_{\odot}}\rm~yr^{-1}$ for 1.5 years. During this event, a fraction of the mass lost by the primary is accreted onto the secondary, with the accretion rate being dependent on the orbital and stellar parameters. From the set of simulations, we derive an analytical relation describing the dependence of the mass accretion rate on the orbital period and stellar mass ratio. We also identify the transitional orbital period for which Roche lobe overflow begins to dominate over wind accretion. We find that accretion leads to a reduction in the effective temperature of the secondary star. However, the mass average accretion rate we obtain in the simulations is low enough for the secondary to remain in thermal equilibrium and avoid radial expansion.

The eccentric short-period O-star binary BD+60 497 is an interesting laboratory to study tidal interactions in massive binary systems, notably via the detection and characterisation of apsidal motion. The rate of apsidal motion in such systems can help us constrain their age and gain insight into the degree of mass concentration in the interior of massive stars. Spectroscopic data collected over two decades are used to reconstruct the individual spectra of the stars and to establish their epoch-dependent radial velocities. An orbital solution, explicitly accounting for apsidal motion is adjusted to the data. Space-borne photometric time series are analysed with Fourier methods and with binary models. We derive a rate of apsidal motion of $6.15^{+1.05}_{-1.65}$ degree/yr which suggests an age of $4.13^{+0.42}_{-1.37}$ Myr. The disentangled spectra unveil a curious change in the spectral properties of the secondary star between the epochs 2002-2003 and 2018-2022 with the secondary spectrum appearing of earlier spectral type over recent years. Photometric data show variability at the 6 mmag level on the period of the binary system which is hard to explain in terms of proximity effects. Whilst the rate of apsidal motion agrees well with theoretical expectations, the changes in the reconstructed secondary spectrum hint at a highly non-uniform surface temperature distribution for this star. Different effects are discussed that could contribute to the photometric variations. The currently most-likely explanation is a mix of proximity effects and tidally excited oscillations

Pulse profile stability in millisecond pulsars (MSPs) is a key factor in achieving high-precision timing essential for detecting nanohertz gravitational waves with Pulsar Timing Arrays (PTAs). In this work, we present a systematic analysis of profile stabilization timescales in MSPs using a direct method based on pulse stacking, applied to long-term multi-epoch observations. Our study utilizes data from the upgraded GMRT (uGMRT) between 300–750 MHz for nine MSPs over 3–5 years and Parkes Ultra-Wideband low-frequency receiver observations (Parkes UWL; covering 704–4032 MHz) for three of them. We find that stable profiles typically require averaging over $10^{5}$–$10^{6}$ pulses. This is the first time such a quantitative approach has been applied to MSPs across a wide frequency range, providing an indirect but practical estimate of jitter noise, a dominant noise source in PTA datasets. We observe that stabilization timescales depend on signal-to-noise ratio, pulse morphology, and surface magnetic field strength, with a moderate correlation indicating a possible role of the magnetic field in emission stability. A complementary single-epoch analysis of nine bright MSPs with uGMRT Band-3 (300–500 MHz) reinforces these results and demonstrates the method's applicability to broader MSP populations. We show that a strong correlation exists between profile-stability slope and the jitter parameter, implying that for faint MSPs, profile-stability analysis can act as an effective proxy for intrinsic pulse-shape variability. Our work provides a novel and scalable framework to assess intrinsic profile variability, helping to guide integration time choices and reduce timing noise in PTA experiments.

We report the discovery of two binary systems, each consisting of a slightly bloated G-type main-sequence star and an unseen companion, identified through photometric data from TESS and radial velocity variation from Gaia. High-resolution spectroscopy confirms orbital periods of 1.37 and 2.67 days with circular orbits. The visible components have masses of $\sim 0.9\,M_\odot$, while the minimum masses of the unseen companions are $1.073^{+0.058}_{-0.060} M_\odot$ and $0.919^{+0.049}_{-0.051} M_\odot$, respectively. Assuming tidal synchronization, we estimate the companion masses to be $1.12^{+0.10}_{-0.08} M_\odot$ and $1.02^{+0.15}_{-0.10} M_\odot$. The absence of detectable spectral features from the companions rules out main-sequence stars of these masses, suggesting that the unseen companions are likely O/Ne or C/O massive white dwarfs. The short orbital periods imply that these systems are post-common envelope binaries. Their subsequent evolution is uncertain, with possible outcomes including cataclysmic variables, Type Ia supernovae, or accretion-induced collapse, depending on the nature of future mass transfer.

The simultaneous observation of gamma rays and neutrinos from the same astrophysical source offers a unique opportunity to probe particle acceleration and interaction mechanisms in ultra-high-energy environments. The Cherenkov Telescope Array Observatory (CTAO) is a next-generation ground-based gamma-ray facility, sensitive to energies from 20 GeV to 300 TeV. In this work, we present for the first time a performance study of CTAO based on joint simulations of steady-state sources emitting both neutrinos and gamma rays, under the assumption that neutrino events are detected by the KM3NeT telescope in the Northern Hemisphere. To identify potentially observable sources, we apply a neutrino-based selection filter according to KM3NeT's discovery potential. We then simulate gamma-ray detectability with CTAO, taking into account visibility, sensitivity, and extragalactic background light absorption. The analysis is specifically focused on exploring the detectability of sources at low neutrino luminosities, limited to values below $10^{52}\,\mathrm{erg\,yr^{-1}}$, in order to assess the performance of CTAO and KM3NeT in identifying faint extragalactic emitters. Particular attention is given to the strategic role of KM3NeT's geographic location, which provides access to Southern-sky sources, and to the impact of the planned CTA+ upgrade, which will enhance CTAO-South with Large-Sized Telescopes (LSTs). Our results highlight the importance of coordinated multi-messenger strategies between KM3NeT and CTAO to maximize the discovery potential of astrophysical neutrino sources.

We present a systematic analysis on the X-ray variability in 13 bright quasars at z > 4.5, combining recent Swift observations from 2021 to 2023 and archival multi-epoch observations. Upper limits of the luminosity measurements were included in the analysis by using the Kaplan-Meier estimator method. It is found that the high-z quasars exhibit X-ray variability on both short-term (hours-to-days) and intermediate-term (weeks-to-months) timescales, with short-term variability dominating the overall variation. A linear correlation exists between the global mean ($\mu_{\mathrm{L_{2-10\,keV}}}$) and standard deviation ($\sigma_{\mathrm{L_{2-10\,keV}}}$) of X-ray luminosities, which is independent of the X-ray photon index and optical-to-X-ray spectral slope. The localized stochastic magnetic reconnection mechanism is strongly favored, which can naturally lead to a scale-invariant power-law energy distribution and satisfactorily explain the correlation. The $\sigma$-$\mu$ correlation parallels with the well-documented rms-flux relation of low-z active galactic nuclei (AGNs), implying the magnetic reconnection mechanism could drive short-timescale X-ray variability in both high- and low-z AGNs. The highest-z quasar in our sample, J142952+544717 (z = 6.18), shows a luminosity distribution extending to ${10}^{47}\ \rm{erg\ {s}^{-1}}$ with a not conspicuous median luminosity. On the other hand, J143023+420436 (z = 4.7), which hosts the most relativistic jet among known high-z blazars, is dominated in the high-luminosity regime (${10}^{47}\ \rm{erg\ {s}^{-1}}$ ), making it an ideal target for multi-wavelength follow-up observations. J090630+693030 is found to have a rest-frame period of 182.46 days and J143023+420436 has a period of 16.89 days, both could be explained by the global evolution of plasmoid chains, in which magnetic islands formed during reconnection may merge successively.

The GeV $\gamma$-ray excess observed towards the Galactic Centre remains unexplained. While dark matter annihilation has long been considered a leading interpretation, an alternative scenario involving a large population of millisecond pulsars has not been ruled out. Testing this hypothesis with electromagnetic observations is difficult, as pulsar searches in the bulge are strongly affected by scattering, high sky temperature, and source confusion. We investigate whether gravitational-wave observations with the Laser Interferometer Space Antenna (LISA) could provide an independent probe of the millisecond pulsar binary population in the Galactic bulge. We construct synthetic populations of millisecond pulsar-white dwarf binaries under two illustrative formation scenarios: an accreted scenario, in which systems are deposited by disrupted globular clusters, and an in situ scenario, in which binaries form through isolated binary evolution. In both cases, only $10^{-5}$–$10^{-4}$ of the underlying bulge population is detectable by LISA. Nevertheless, even a few detections would imply tens to hundreds of thousands of unseen systems. Accreted binaries are expected to have lower chirp masses ($\sim$0.4 M$_\odot$), while in situ binaries produce more massive companions ($\sim$0.9 M$_\odot$). LISA will measure binary frequencies with high precision, but chirp masses can only be determined for the most massive or highest-frequency systems. Distinguishing millisecond pulsar binaries from the far more numerous double white dwarfs will be challenging, though LISA detections could provide valuable targets for follow-up with the Square Kilometre Array, enabling a critical test of the millisecond pulsar origin of the $\gamma$-ray excess.

The IceCube Neutrino Observatory, situated at the geographic South Pole, comprises both a surface component, IceTop, and a deep in-ice component. This unique setup allows for simultaneous measurements of low-energy ($\sim \rm{GeV}$) and high-energy ($\gtrsim 400\,\rm{GeV}$) muons generated in cosmic-ray air showers. The correlation between these low- and high-energy muons can serve as a valuable tool not only for analyzing cosmic-ray composition but also for tests of hadronic interaction models. However, IceTop does not feature dedicated muon detectors, making it challenging to measure the low-energy muon component for individual air showers. \\ \noindent For this reason, a two-component lateral distribution function is utilized for the simultaneous reconstruction of the primary energy and low-energy muon number on a single-event basis. This is achieved by combining analytical descriptions of the electromagnetic and muon lateral distributions. In this work, the underlying principles of this method will be discussed, as well as its capability for muon number reconstruction using the hadronic interaction models Sibyll 2.1, QGSJet-II.04, and EPOS-LHC.

The IceCube Neutrino Observatory actively participates in multimessenger follow-ups of gravitational-wave (GW) events. With the release of the Gravitational-Wave Transient Catalogue (GWTC)-2.1 and -3, the sub-threshold GW event information from the third observation run of the LIGO-Virgo-KAGRA (LVK) detectors is publicly available. These sub-threshold GWs are identified via template-based and minimally modelled search pipelines. Neutrino counterparts can enhance their astrophysical significance and improve their localisation. In this contribution, we propose a catalogue-based search for sub-TeV neutrino counterparts to sub-threshold GWs. For this search, we use archival data from IceCube's dense infill array, DeepCore. Using the unbinned maximum likelihood method, we search for correlation between IceCube sub-TeV neutrinos and the $\sim$ 100 most significant sub-threshold GW source candidates. With this study, we aim to contribute to the ongoing efforts to identify common astrophysical sources of neutrinos and GWs. We present the current status of this search and its role in advancing multimessenger astronomy, paving the way for deeper exploration of GW events and their sources.

Massive stars are often born in triples, where gravitational dynamics and stellar interactions play a crucial role in shaping their evolution. One such pathway includes the merger of the inner binary, transforming the system to a binary with a distinct formation history. Therefore, the interpretation of observed binary properties and their inferred formation history may require the consideration of a potential triple origin. We aim to investigate the population of stellar mergers in massive hierarchical triples. Specifically, we assess how frequently mergers occur, and characterise the properties of the post-merger binaries and their subsequent evolution. We combine the triple population synthesis code TRES, which self-consistently models stellar evolution, binary interaction, and gravitational dynamics, with the binary population synthesis code SeBa to simulate 10^5 dynamically stable, massive triples from the zero-age main sequence through merger and post-merger evolution. We explore the effects of a range of physical models for the initial stellar properties, mass transfer, and merger. We find that stellar mergers are a common outcome, occurring in 20-32% of massive triples. Most mergers happen relatively early in the evolution of the system and involve two main-sequence (MS) stars, producing rejuvenated merger remnants that can appear significantly younger than their tertiary companions. Consequently, we predict that 2-10% of all wide MS+MS binaries (P>100 days) have a measurable age discrepancy, and serve as a promising way to identify merged stars. The post-merger systems preferentially evolve into wide, eccentric binaries, with  80% avoiding further interaction. However, a notable fraction (16-22%) undergoes a second mass-transfer phase, which may result in the formation of high-mass X-ray binaries or mergers of compact objects that spiral in via gravitational-wave emission.

Coronal loops are plasma structures in the solar atmosphere with temperatures reaching millions of Kelvin, shaped and sustained by the magnetic field. However, their morphology and fundamental nature remain subjects of debate. By studying their cross-sectional properties and how they change along the loop and in time, we can understand their magnetic structure and heating mechanisms. In this study, we investigated the cross-sectional intensity profiles, both spatially and temporally, of two unique coronal loops, observed in the periphery of two distinct active regions by the Extreme Ultraviolet Imager (EUI/HRI$_{\rm EUV}$) on board the Solar Orbiter spacecraft. The main results of this study are 1. The lifetimes of these two loops (loop1 > 120 min & loop2 > 50 min) are longer than the typical timescales of radiative cooling and thermal conduction. 2. Their widths determined by the FWHM of the single Gaussian fit to the cross-axis intensity profiles are greater than 6-7 pixels of EUI/HRI$_{\rm EUV}$, indicating that the loop cross-section is uniformly filled on well-resolvable scales. 3. These loops exhibited an almost constant width, both spatially and temporally (width for loop1 is 2.1 $\pm$ 0.4 Mm and for loop2 is 1.3 $\pm$ 0.2 Mm), indicating that they are stable non-expanding structures. 4. We present observational evidence that the one of the loops (loop2) is not braided, which strongly suggests that the non-expanding nature of this multi-stranded loop along its length cannot be attributed to the twist of the magnetic field lines. In conclusion, we find that these coronal loops are long, stable, multi-stranded, non-expanding structures with a uniform cross-section that persist in the corona for an unusually extended duration. This not only challenges our current understanding of the structure of the coronal magnetic field but also raises critical questions about the mechanisms

The vertical distribution of pebbles in protoplanetary disks is a fundamental property influencing planet formation, from dust aggregation to the assembly of planetary cores. In the outer region of protoplanetary disks, the intensity of the optically thin but geometrically thick dust ring decreases along the minor axis due to reduced line-of-sight optical depth. Multi-ring disks thus provide an excellent opportunity to study the radial variation of the vertical properties of dust. We investigate the vertical dust distribution in 6 protoplanetary disks with resolved double rings, using high-resolution ALMA Band 6 continuum observations. By modeling the azimuthal intensity variations in these rings, we constrain the dust scale heights for each ring. Our results reveal a dichotomy: inner rings exhibit puffed-up dust layers with heights comparable to the gas scale height, while outer rings are significantly more settled, with dust scale heights less than 20\% of the gas scale height. This suggests a radial dependence in dust settling efficiency within the disks, potentially driven by localized planetary interactions or the global radial dependence of the Vertical Shear Instability (VSI). We discuss the implications of these findings for dust trapping, planet formation, and protoplanetary disk evolution. Our work highlights the importance of vertical dust distribution in understanding the early stages of planet formation and suggests that outer ($>80$ au), settled rings are preferred sites for planet formation over inner ($<80$ au), turbulent rings.

Pulsars and their pulsar wind nebulae (PWNe) are unique laboratories for extreme astrophysical processes. This dissertation combines Fermi-LAT observations with a time-dependent leptonic PWN model (TIDE) to explore their gamma-ray emission. The model was validated on benchmark PWNe and applied to a systematic LAT search, leading to the discovery of new MeV-GeV candidates and the first population-level characterization. Several sources were modeled in detail, and predictions for potential TeV-emitting PWNe were tested against current and future observatories. Ultra-high-energy gamma-ray sources were also examined, revealing limits of standard leptonic scenarios. Beyond PWNe, stringent upper limits were obtained for the binary pulsar 1A 0535+262, and steady emission was detected from the globular cluster M5. Together, these results advance our understanding of pulsar environments and establish a framework for future multi-wavelength and next-generation gamma-ray studies.

Detecting parity violation on cosmological scales would provide a striking clue to new physics. Large-scale structure offers the raw statistical power – many three-dimensional modes – to make such tests. However, for scalar observables, like galaxy clustering, the leading parity-sensitive observable is the trispectrum, whose high dimensionality makes the measurement and noise estimation challenging. We present two late-time parity-odd kurto spectra that compress the parity-odd scalar trispectrum into one-dimensional, power-spectrum-like observables. They are built by correlating (i) two appropriately weighted quadratic composite fields, or (ii) a linear and cubic composite field, constructed from dark matter (DM) or galaxy overdensity fields. We develop an FFTLog pipeline for efficient theoretical predictions of the two observables. We then validate the estimators for a specific parity-odd primordial template on perturbative DM field, and on DM and halo fields in full N-body \texttt{Quijote} simulations, with and without parity-odd initial conditions, in real and redshift space. For DM, the variance is dominated by the parity-even contribution – i.e., the gravitationally induced parity-even trispectrum – and is efficiently suppressed by phase-matched fiducial subtraction. For halos, discreteness-driven stochasticity dominates and is not appreciably reduced by subtraction; however, optimal weighting and halo-matter cross kurto spectra considerably mitigate this noise and enhance the signal. Using controlled down-sampling of the matter field, we empirically calibrate how the parity-even variance scales with number density and volume, and provide an illustrative forecast for the detectability of parity-odd kurto spectra in a Euclid-like spectroscopic galaxy survey.

DES-5Y supernovae, combined with DESI BAO, appear to favour Chevallier-Polarski-Linder $(w_0, w_a)$ dynamical dark energy over $\Lambda$CDM. arXiv:2408.07175 suggested that this is driven by a systematic in the DES pipeline, which particularly affects the low-redshift supernovae brought in from legacy surveys. It is difficult to investigate these data in isolation, however, as the complicated supernovae pipelines must properly account for selection effects. In this work, we discover that the Bayesian evidence previously found for flexknot dark energy ( arXiv:2503.17342 ) is beaten by a magnitude offset between the low- and high-redshift supernovae. In addition, we find that the possible tension between DES-5Y and DESI is significantly reduced by such an offset. We also take the opportunity to trial Nested Bridge Sampling with Sequential Monte Carlo as an alternative method for calculating Bayes factors.

Searching for exomoons is attempted via Kepler and TESS, but none is confirmed. Theoretically, similar with Jupiter, the gas giants are possible to generate moons. However, HJs which are considered to form outside and then move close to the star are thought not easy to sustain the original moons via dynamical effects. In this paper, we assume the HJ to form at 1 AU and move inward via disk migration or migration due to planet secular coplanar. Then we simulate the dynamics of exomoon-planet systems during migration, and we want to study the fates of different original moons. We find that both prograde and retrograde moons could maintain stable after disk migration, although the retained fraction of retrograde moons is 5 times higher than the prograde moons. Only massive and retrograde moons (greater than 10 Earth masses) might survive around HJs during the coplanar excitation. Furthermore, 6\% of the original Jupiter-like planet can also form free-floating planets after undergoing coplanar excitation, and most of them retain their moons. Our results focus on the fate of the exomoons and provide a clue on where to find the moon for future missions.

The primary radiation from thermonuclear X-ray bursts observed in the neutron star low-mass X-ray binary (LMXB) systems can interact with various parts of the binary system. This interaction gives rise to secondary radiation in different wavelength ranges, known as reprocessed emission. In eclipsing LMXBs, the reprocessed emission from the bursts can be examined during eclipses, as the primary emission is blocked and only the reprocessed emission is visible. We searched for bursts during eclipses in the archival RXTE data of the eclipsing LMXBs and found them in EXO 0748-676 and XTE J1710-281. In EXO 0748-676, seven bursts were found to occur near eclipse egress, with their tails extending beyond the eclipse, and one such burst was found for XTE J1710-281. We estimate the reprocessing fraction at orbital phases near eclipse egress by modeling the peculiar eclipse bursts detected in both systems, which have tails extending beyond the eclipses. We observe an increasing trend in reprocessing fraction as these eclipse bursts occur closer to the eclipse egress. We discuss the possibilities of reprocessing in the ablated wind from the companion star, the accretion disc, and the disc wind in EXO 0748-676 and XTE J1710-281. Additionally, we observe two decay components in the bursts in EXO 0748-676, which could suggest a complex composition of the accreting fuel. From the burst rise timescales, we place an upper limit on the size of the reprocessing regions in both EXO 0748-676 and XTE J1710-281, finding it comparable to the size of the respective X-ray binaries.

In this work we examine the 2025 DESI analysis of dark energy, which suggests that dark energy is evolving in time with an increasing equation of state $w$. We explore a wide range of quintessence models, described by a potential function $V(\varphi)$, including: quadratic potentials, quartic hilltops, double wells, cosine functions, Gaussians, inverse powers. We find that while some provide improvement in fitting to the data, compared to a cosmological constant, the improvement is only modest. We then consider non-minimally coupled scalars which can help fit the data by providing an effective equation of state that temporarily obeys $w<-1$ and then relaxes to $w>-1$. Since the scalar is very light, this leads to a fifth force and to time evolution in the effective gravitational strength, which are both tightly constrained by tests of gravity. For a very narrow range of carefully selected non-minimal couplings we are able to evade these bounds, but not for generic values.

We demonstrate a GPU-accelerated nested sampling framework for efficient high-dimensional Bayesian inference in cosmology. Using JAX-based neural emulators and likelihoods for cosmic microwave background and cosmic shear analyses, our approach provides parameter constraints and direct calculation of Bayesian evidence. In the 39 dimensional $\Lambda$CDM vs $w_0w_a$ shear analysis, we produce Bayes Factors and a robust error bar in just 2 days on a single A100 GPU, without loss of accuracy. Where CPU-based nested sampling can now be outpaced by methods relying on MCMC sampling and decoupled evidence estimation, we demonstrate that with GPU acceleration nested sampling offers the necessary speed-up to put it on equal computational footing with these methods, especially where reliable model comparison is paramount. We put forward both nested and gradient-based sampling as useful tools for the modern cosmologist, where cutting-edge inference pipelines can yield orders of magnitude improvements in computation time.

Recent measurements of baryon acoustic oscillations (BAO) from the Dark Energy Spectroscopic Instrument (DESI) have been interpreted to suggest that dark energy may be evolving. In this work, we examine how prior choices affect such conclusions. Specifically, we study the biases introduced by the customary use of uniform priors on the Chevallier-Polarski-Linder (CPL) parameters, $w_0$ and $w_a$, when assessing evidence for evolving dark energy. To do so, we construct theory-informed priors on $(w_0, w_a)$ using a normalizing flow (NF), trained on two representative quintessence models, which learns the distribution of these parameters conditional on the underlying $\Lambda$CDM parameters. In the combined $\textit{Planck}$ CMB + DESI BAO analysis we find that the apparent tension with a cosmological constant in the CPL framework can be reduced from $\sim 3.1\sigma$ to $\sim 1.3\sigma$ once theory-informed priors are applied, rendering the result effectively consistent with $\Lambda$CDM. For completeness, we also analyze combinations that include Type Ia supernova data, showing similar shifts toward the $\Lambda$CDM limit. Taken together, the observed sensitivity to prior choices in these analyses arises because uniform priors - often mischaracterized as "uninformative" - can actually bias inferences toward unphysical parameter regions. Consequently, our results underscore the importance of adopting physically motivated priors to ensure robust cosmological inferences, especially when evaluating new hypotheses with only marginal statistical support. Lastly, our NF-based framework achieves these results by post-processing existing MCMC chains, requiring $\approx 1$ hour of additional CPU compute time on top of the base analysis - a dramatic speedup over direct model sampling that highlights the scalability of this approach for testing diverse theoretical models.

We demonstrate that Gaia's detection of stars on wide orbits around black holes opens a new observational window on dark matter structures – such as scalar clouds and dark matter spikes – predicted in a range of theoretical scenarios. Using precise radial velocity measurements of these systems, we derive state-of-the-art constraints on dark matter density profiles and particle masses in previously unexplored regions of parameter space. We also test the black hole hypothesis against the alternative of a boson star composed of light scalar fields.

We present the first evolving interior structure model for sub-Neptunes that accounts for the miscibility between silicate magma and hydrogen. Silicate and hydrogen are miscible above $\sim 4000$K at pressures relevant to sub-Neptune interiors. Using the H$_2$-MgSiO$_3$ phase diagram, we self-consistently couple physics and chemistry to determine the radial extent of the fully miscible interior. Above this region lies the envelope, where hydrogen and silicates are immiscible and exist in both gaseous and melt phases. The binodal surface, representing a phase transition, provides a physically/chemically informed boundary between a planet's "interior" and "envelope". We find that young sub-Neptunes can store several tens of per cent of their hydrogen mass within their interiors. As the planet cools, its radius and the binodal surface contract, and the temperature at the binodal drops from $\sim 4000$K to $\sim 3000$K. Since the planet's interior stores hydrogen, its density is lower than that of pure-silicate. Gravitational contraction and thermal evolution lead to hydrogen exsolving from the interior into the envelope. This process slows planetary contraction compared to models without miscibility, potentially producing observable signatures in young sub-Neptune populations. At early times ($\sim 10$-$100$Myr), the high temperature at the binodal surface results in more silicate vapour in the envelope, increasing its mean molecular weight and enabling convection inhibition. After $\sim$Gyr of evolution, most hydrogen has exsolved, and the radii of miscible and immiscible models converge. However, the internal distribution of hydrogen and silicates remains distinct, with some hydrogen retained in the interior.

Four-derivative heterotic supergravity (without gauge fields) reduced on a $p$-dimensional torus leads to half-maximal supergravity coupled to $p$ vector multiplets, and it is known that removing the vector multiplets is a consistent truncation of the theory. We find a new consistent truncation of four-derivative heterotic supergravity on a torus that keeps the vector multiplets and precisely reproduces the bosonic action of heterotic supergravity (with heterotic gauge fields). We show that both truncations have an $O(d+p,d)$ symmetry when reduced on a $d$-dimensional torus and demonstrate how this embeds in the $O(d+p,d+p)$ symmetry that one gets from reducing on a $(d+p)$-dimensional torus without truncation. We then use our new truncation to obtain four-derivative corrections to the Kerr-Sen solution and compute thermodynamic quantities and multipole moments. Finally, we compare the Kerr-Sen solutions of the actions corresponding to the two different choices of truncation with the Kerr solution, the Kerr-Newman solution, and each other, and show that they have distinct four-derivative multipole structures.

LHAASO, a ground-based observatory, is unveiling new frontiers in our understanding of high-energy $\gamma-$rays and cosmic rays. It has recently observed high energy diffuse $\gamma-$rays from the Galactic plane in the TeV-PeV range. For the first time, we analyze this data to search for signatures of heavy decaying and annihilating dark matter in the mass range $10^{5}-10^{11}$ GeV. We compute the expected photon flux from both Galactic and extragalactic dark matter, incorporating attenuation due to photon pair production. For the Galactic contribution, we include both prompt photons and secondary photons produced via inverse Compton scattering, accounting for electron/positron propagation. For the extragalactic component, in addition to the prompt and inverse Compton contributions, we also include cascade photons arising from inverse Compton scattering of pair-produced electrons and positrons. By combining all these contributions, we derive constraints on the dark matter parameter space. Our bounds for various two body Standard Model final states are strongest to date. This underscore LHAASO's capability to discover the nature of heavy dark matter.

Bumblebee models, a class of vector-tensor theories in which a vector field acquires a nonzero vacuum expectation value that spontaneously breaks spacetime symmetries, are ubiquitous in the literature. In this paper, we highlight several often-overlooked properties of these models by analyzing their cosmological perturbations. We show that a non-minimal coupling to gravity is essential for the stability of the setup. However, avoiding propagation of a ghost mode then requires imposing a relation between the coupling coefficients, known as the degeneracy condition, which reduces the bumblebee model to a subset of generalized Proca theories with a marginal non-minimal operator. By imposing the degeneracy condition, the vector field becomes non-dynamical at the background level, and the form of its potential is completely fixed in vacuum. We show that the vacuum expectation value of the vector field can drive a de Sitter solution, for which the effects of the non-minimal coupling are negligible at the background level but provide essential order-one corrections to the sound speed of the scalar mode, keeping the setup weakly coupled at the level of perturbations. Treating this stealth de Sitter solution as a dark energy candidate, we study its coupling to matter and find the effective gravitational coupling for the matter density contrast in the quasi-static regime. At the level of perturbations, the system behaves differently from $\Lambda$CDM, providing a potential observational signature to distinguish the two models.

Inflationary production of massive dark photons with non-minimal couplings to gravity shows surprising growth at large momenta. These couplings appear in the effective low energy description of a more fundamental theory. We find that the growth is absent in explicit gauge invariant UV-complete models. Such completions are also free of "ghost" instabilities, which often appear in the effective models.

We propose a minimal extension of the type-I seesaw model to realise leptogenesis from the co-annihilation of dark sector particles. The type-I seesaw model is extended with a singlet fermion and two singlet scalars charged under a $Z_{2}$ symmetry. The $Z_{2}$-odd singlet scalar is the dark matter candidate. Here the usual type-I seesaw mechanism generates neutrino mass, and a net lepton asymmetry is generated from the co-annihilation of the dark matter and the $Z_2$-odd singlet fermion. The $Z_{2}$-even singlet scalar is important in dark matter phenomenology. Successful leptogenesis is possible at TeV-scale, unlike the vanilla case. This minimal extension provides an elegant explanation of successful leptogenesis with direct connection to the dark matter abundance in the Universe.

We study the production of primordial gravitational waves (GWs) from first-order phase transitions (FOPTs) in extensions of the Standard Model based on Flavour Deconstruction (FD). The link fields inherent to FD generically form a rich scalar sector, with sizeable couplings at the TeV scale, providing natural conditions for strong FOPTs and correspondingly large GW emission. We identify the key parameters controlling the GW spectrum and enabling its detection at future GW observatories. In particular, we find that while FD scenarios can yield detectable signals, the resulting spectra typically peak at higher frequencies than the millihertz range. As a consequence, a positive observation at LISA is possible but not guaranteed, while the signal falls in the range of mid-band proposals, making FD models an intriguing target for upcoming GW searches.

The anisotropic influence on the $f$-mode frequency of oscillations and dimensionless tidal deformability of strange quark matter are analyzed by employing the nonradial oscillation equations for the complete general relativity frame and tidal deformability equations, which are derived and modified from their standard form to introduce the anisotropic factor. The fluid in the compact star follows the MIT bag model with vector coupling. For the anisotropic function, we use a local anisotropy, which is regular along the whole star and is null both at the center and on the star's surface. We show that the $f$-frequency of oscillation and dimensionless tidal deformability change considerably with the anisotropy. Finally, we investigate the correlation between the dimensionless tidal deformability of the GW$170817$ event with the anisotropy.

We consider the behavior of the analogue of the Lemaitre time when a particle approaches the horizon of a rotating black hole. For the Kerr metric, the aforementioned time coincides with the Doran or Natario time but we consider a more general class of metrics. We scrutiny relationship between (i) its finiteness or divergence, (ii) the forward-in-time condition, (iii) the sign of a generalized momentum/energy, (iv) the validity of the principle of kinematic censorship. The latter notion means impossibility to release in any event an energy which is literally infinite. As a consequence, we obtain a new explanation, why collisions of two particles inside the horizon do not lead to infinite energy in their center of mass frame. The same results are also obtained for the Reissner-Nordström metric

I revisit whether black-hole remnants, from sub-Planckian compact objects to Planck relics and up to (super)massive black holes, can preserve Standard-Model (SM) electric charge. Two exterior-field mechanisms – Coulomb-focused capture from ambient media and QED Schwinger pair production – robustly neutralize such objects across cosmic history. I first derive the general capture rate including both Coulomb and gravitational focusing, and sum the stepwise discharge time in closed form via the trigamma function, exhibiting transparent Coulomb- and gravity-dominated limits. I then integrate the Schwinger rate over the near-horizon region to obtain an explicit $\dot Q(Q)$ law: discharge proceeds until the horizon field falls below $E_{\rm crit}$, leaving a residual charge $Q_{\rm stop}^{(e)}\!\propto\! r_h^2$ that is $\ll e$ for Planck radii. Mapping the mass dependence from sub-Planckian to astrophysical scales, I also analyze dark-sector charges with heavy carriers (including kinetic mixing and massive mediators). In a conservative “no-Schwinger” limit where vacuum pair creation is absent, cumulative ambient exposures alone force discharge of any integer SM charge. Three possible loopholes remain. (i) A fine-tuned SM corner in which the relic sits arbitrarily close to Reissner-Nordström extremality so greybody factors suppress charged absorption, while Schwinger pair creation is absent due to Planck-scale physics. (ii) Charge relocated to a hidden $U(1)_D$ with no light opposite carriers, e.g. if the lightest state is very heavy and/or kinetic mixing with $U(1)_{\rm EM}$ is vanishingly small. (iii) Discrete or topological charges rather than ordinary SM electric charge. Outside these cases, the conclusion is robust: within SM electromagnetism, charged black-hole relics neutralize efficiently and cannot retain charge over cosmological times.

Magnetic reconnection is a fundamental and omnipresent energy conversion process in plasma physics. Novel observations of fields and particles from Parker Solar Probe (PSP) have shown the absence of reconnection in a large number of current sheets in the near-Sun solar wind. Using near-Sun observations from PSP Encounters 4 to 11 (Jan 2020 to March 2022), we investigate whether reconnection onset might be suppressed by velocity shear. We compare estimates of the tearing mode growth rate in the presence of shear flow for time periods identified as containing reconnecting current sheets versus non-reconnecting times, finding systematically larger growth rates for reconnection periods. Upon examination of the parameters associated with reconnection onset, we find that 85% of the reconnection events are embedded in slow, non-Alfvenic wind streams. We compare with fast, slow non-Alfvenic, and slow Alfvenic streams, finding that the growth rate is suppressed in highly Alfvenic fast and slow wind and reconnection is not seen in these wind types, as would be expected from our theoretical expressions. These wind streams have strong Alfvenic} flow shear, consistent with the idea of reconnection suppression by such flows. This could help explain the frequent absence of reconnection events in the highly Alfvenic, near-Sun solar wind observed by PSP. Finally, we find a steepening of both the trace and magnitude magnetic field spectra within reconnection periods in comparison to ambient wind. We tie this to the dynamics of relatively balanced turbulence within these reconnection periods and the potential generation of compressible fluctuations.

We investigate the equation of state (EOS) and macroscopic properties of neutron stars (NSs) and hyperonic stars within the framework of the lowest order constrained variational (LOCV) method, extended to include interacting $\Lambda$ hyperons. The nucleon-nucleon interaction is modeled using the AV18 potential supplemented by Urbana three-body forces, while $\Lambda N$ and $\Lambda \Lambda$ interactions are described by realistic spin- and parity-dependent potentials fitted to hypernuclear data. Cold, charge-neutral, and $\beta$-equilibrated matter composed of neutrons, protons, electrons, muons, and $\Lambda$ hyperons is considered. We compute particle fractions, chemical potentials, the EOS, speed of sound, tidal deformability, and stellar structure by solving the Tolman-Oppenheimer-Volkoff equations, and compare our results with recent NICER and gravitational-wave observations. The inclusion of $\Lambda$ hyperons leads to EOS softening, reducing the maximum NS mass from $2.34M_\odot$ to $2.07M_\odot$, while keeping it consistent with the $2M_\odot$ mass constraint. At $1.4M_\odot$, the model satisfies observational limits on radius and tidal deformability, with the $\Lambda$ onset occurring below this mass. Comparison with other microscopic and relativistic mean-field models shows that our EOS remains consistent with the allowed pressure-energy density range, while also permitting even canonical-mass NSs of about $1.4M_{\odot}$ to accommodate hyperons. These results suggest that hyperons can appear in NSs across the observed mass range without violating current astrophysical constraints, and that the extended LOCV method provides a consistent, microscopic approach to modeling dense hypernuclear matter.

This dissertation focuses on the reconstruction of Equations of State (EoSs) describing the interior of compact stars, using modern machine learning and deep learning methods. The pipeline is based on data from mass-radius (M-R) curves, obtained by numerically solving the Tolman-Oppenheimer-Volkoff equations for a wide range of admissible EoSs. The manuscript is divided into a Theoretical Part (Chs. 1-4) and a Computational Part (Chs. 5-7). The theoretical chapters analyze the properties of neutron and quark stars, the physical constraints of viable EoS models, and introduce regression algorithms (Decision Tree, Random Forest, Gradient Boosting, XGBoost) and neural networks with normalization and dropout techniques. The computational part presents the generation of artificial EoSs for hadronic and quark stars (MIT bag, CFL), the numerical solution of the TOV equations, data preparation, and hyperparameter tuning. Results include training and evaluation of models using MSE/MSLE metrics, learning curves for neural networks, and reconstruction of 21 hadronic and 20 quark star EoSs. Source code and tools for reproducibility and future research are provided. The work aims to establish a reusable and scalable framework, strengthening the connection between theoretical astrophysics and computational science.

The isotopic ratios measured in meteoritic presolar grains are a crucial tool for tracing the nucleosynthetic origin of isotopes. In the case of silicon isotopes, two important indicators to establish the origin of presolar grains are the ratios 29Si/28Si and 30Si/28Si. To constrain theoretical predictions, the rates of key nuclear reactions influencing the abundances of 29Si and 30Si must be well known. One such reaction is 29Si(p,gamma)30P which plays a role in classical nova explosions. The aim of the present work is to determine the nonresonant cross section of the 29Si(p,gamma)30P reaction, which has not been previously measured. The activation method was employed to measure the total cross section at four proton energies between Ep = 1000 and 1430 keV. The measured cross sections were found to be significantly (a factor of 4.3+-0.6) higher than those predicted by theoretical direct capture calculations, thereby impacting the reaction rates at low astrophysical temperatures, below about 30 MK. This higher nonresonant cross section - now based on experimental data - can be used in forthcoming nucleosynthesis calculations of classical novae. As a secondary result, the 16O(p,gamma)17F cross section was also obtained and found to be in good agreement with existing literature data.

We show that a simple supersymmetric $U(1)_{B-L}$ extension of the standard model can explain simultaneously the large electron neutrino asymmetry hinted by the recent EMPRESS data as well as the observed tiny baryon number asymmetry via the resonant leptogenesis mechanism. The condensation of $B-L$ Higgs dominating the universe at its decay is the sole source for these generation processes. Here, the infrequent decays of the $B-L$ Higgs to heavy right handed neutrinos and successive prompt decays of these right handed neutrinos around the electroweak phase transition produce the observed baryon number asymmetry, while the complete decay of the same $B-L$ Higgs at a later epoch leads to a large lepton number asymmetry. The right amounts of both asymmetries are found to be obtained for the symmetry-breaking scale $v_\phi \sim 10^{10}~{\rm GeV}$. Moreover, in a close connection to the positivity of both asymmetries, seemingly only the normal mass hierarchy of light neutrino species works. Finally, the gravitational wave background from the topologically stable strong type-I cosmic strings, generated from the breaking of $U(1)_{B-L}$ symmetry, can be within the reach of future experiments such as ultimate DECIGO.

The macroscopic model for a neutron star (NS) as a perfect liquid drop at the equilibrium is extended to rotating systems with a small frequency $\omega$ within the effective-surface (ES) approach. The NS angular momentum $I$ and moment of inertia (MI) for a slow stationary azimuthal rotation around the symmetry axis is calculated by using the Kerr metric approach in the Boyer-Lindquist and Hogan forms for the perfect liquid-drop model of NSs. The gradient surface terms of the NS energy density $\mathcal{E}(\rho)$ [Equation of State] are taken into account along with the volume ones at the leading order of the leptodermic parameter $a/R \ll 1$, where $a$ is the ES crust thickness and $R$ is the mean NS radius. The macroscopic NS angular momentum $I$ at small frequencies $\omega$, up to quadratic terms, can be specified for calculations of the adiabatic MI, $\Theta=d I/d \omega$, by using Hogan's inner gravitational metric, $r\le R$. The NS MI, $\Theta=\tilde{\Theta}/(1-\mathcal{G}_{t\varphi})$, was obtained in terms of the statistically averaged MI, $\tilde{\Theta}$, and its time and azimuthal angle correlation, $\mathcal{G}_{t\varphi}$, as sumes of the volume and surface components. The MI $\Theta$ depends dramatically on its effective radius $R$ because of a strong gravitation. We found the significant shift of the Schwarzschild radius $R_{\rm S}$ to a much smaller position due to the time and azimuthal correlation term $\mathcal{G}_{t\varphi}$. The adiabaticity condition is carried out for several neutron stars in a strong gravitation case.

We develop the Kazantsev theory of small-scale dynamo generation at small Prandtl numbers near the generation threshold and restore the concordance between the theory and numerical simulations: the theory predicted a power-law decay below the threshold, while simulations demonstrate exponential decay. We show that the exponential decay is temporary and owes its existence to the flattening of the velocity correlator at large scales. This effect corresponds to the existence of a long-living virtual level in the corresponding Schrodinger type equation. We also find the critical Reynolds number and the increment of growth/decay above and under the threshold; we express them in terms of the quantitative characteristic properties of the velocity correlator, which makes it possible to compare the results with the data of different simulations.

Recently, two of the present authors showed that even when the axion momentum is much smaller than its mass, the axion can still behave like radiation if its energy density greatly exceeds the maximum potential energy set by the cosine-type potential. As the energy density redshifts down to the potential scale, a nonlinear transition occurs, during which the axion's adiabatic invariant is not conserved. In this paper, we revisit the analysis of axion dark matter by incorporating the effects of this nonlinear transition through a precise study of the axion spectrum. We demonstrate that in the parameter region with a relatively small decay constant, often favored in axion search experiments, special care is required when estimating the axion abundance and spectrum. We also highlight a scenario in which axions are produced through the stimulated decay of a modulus, a situation that may naturally arise in the string axiverse, where the nonlinear transition occurs across a wide parameter region. Furthermore, we discuss related phenomena, including QCD axion dark matter, the formation of axion clumps such as miniclusters and axion stars, gravitational wave production, and formation of primordial black holes as dark matter.

We study magnetic conversion of ultra-relativistic axion-like particles (ALPs) into photons in compact-star environments, focusing on the hot, transient conditions of core-collapse supernova (SN) remnants and neutron-star mergers (NSMs). We address previously overlooked uncertainties, particularly the suppression caused by ejected matter near the stellar surface, a region crucial to the conversion process. We derive analytical expressions for the transition rate; they reveal the influence of key parameters and their uncertainties. We update constraints using historical gamma-ray data from SN 1987A and find $g_{a\gamma}<5\times10^{-12}~{\rm GeV}^{-1}$ for $m_a\lesssim10^{-9}$ meV. We also forecast sensitivities for a future Galactic SN and for NSMs, assuming observations with Fermi-LAT or similar gamma-ray instruments. We distinguish ALPs – defined as coupling only to photons and produced via Primakoff scattering – from axions, which also couple to nucleons and emerge through nuclear bremsstrahlung. We omit pionic axion production due to its large uncertainties and inconsistencies, though it could contribute comparably to bremsstrahlung under optimistic assumptions. For the compact sources, we adopt time-averaged one-zone models, guided by numerical simulations, to enable clear and reproducible parametric studies.

We introduce OneCovariance, an open-source software designed to accurately compute covariance matrices for an arbitrary set of two-point summary statistics across a variety of large-scale structure tracers. Utilising the halo model, we estimated the statistical properties of matter and biased tracer fields, incorporating all Gaussian, non-Gaussian, and super-sample covariance terms. The flexible configuration permits user-specific parameters, such as the complexity of survey geometry, the halo occupation distribution employed to define each galaxy sample, or the form of the real-space and/or Fourier space statistics to be analysed. We illustrate the capabilities of OneCovariance within the context of a cosmic shear analysis of the final data release of the Kilo-Degree Survey (KiDS-Legacy). Upon comparing our estimated covariance with measurements from mock data and calculations from independent software, we ascertain that OneCovariance achieves accuracy at the per cent level. When assessing the impact of ignoring complex survey geometry in the cosmic shear covariance computation, we discover misestimations at approximately the $10\%$ level for cosmic variance terms. Nonetheless, these discrepancies do not significantly affect the KiDS-Legacy recovery of cosmological parameters. We derive the cross-covariance between real-space correlation functions, bandpowers, and COSEBIs, facilitating future consistency tests among these three cosmic shear statistics. Additionally, we calculate the covariance matrix of photometric-spectroscopic galaxy clustering measurements, validating the jackknife covariance estimates for calibrating KiDS-Legacy redshift distributions. The OneCovariance can be found on GitHub, together with comprehensive documentation and examples.

A number of important cosmological questions can be addressed only by probing perturbation modes on the largest accessible scales. One promising probe of these modes is the Kamionkowski-Loeb effect, i.e., the polarization induced in the cosmic microwave background (CMB) by Thomson scattering in galaxy clusters, which is proportional to the CMB quadrupole measured at the cluster's location and look-back time. We develop a Fisher formalism for assessing the amount of new information that can be obtained from a future remote quadrupole survey. To demonstrate the constraining power of such a survey, we apply our formalism to a model that suppresses the primordial power spectrum on large scales but is poorly constrained with existing CMB data. We find that the constraints can be improved by over $3\sigma$ for a survey that measures around 100 clusters over $20\%$ of the sky with a signal-to-noise ratio of $3$. In the most optimistic case with a low-noise survey with dense full-sky coverage and only a single degree of freedom in the theory, the constraint improves to over $7\sigma$ beyond local CMB data. Our formalism, which is based in real space rather than harmonic space, can be used to explore a wide range of survey designs, and our results paint an optimistic picture for the utility of remote quadrupole measurements to probe physics on the largest observable scales in the Universe.

The Axelrod approximation is widely used in astrophysical modelling codes to evaluate electron-impact excitation effective collision strengths for forbidden transitions. Approximate methods such as this are a necessity for many heavy elements with open shells where collisional data is either non existent or sparse as the use of more robust methods prove prohibitively expensive. Atomic data for such forbidden transitions are essential for producing full collisional radiative models that do not assume Local-Thermodynamic-Equilibrium (LTE). In this short work we repeat the optimization of the simple Axelrod formula for a large number of $R$-matrix data sets, ranging from Fe and Ni to the first r-process peak elements of Sr, Y and Zr, to higher Z systems Te, W, Pt and Au. We show that the approximate treatment of forbidden transitions can be a significant source of inaccuracy in such collisional radiative models. We find a large variance of the optimized coefficients for differing systems and charge states, although some general trends can be seen based on the orbital structure of the ground-state-configurations. These trends could potentially inform better estimates for future calculations for elements where $R$-matrix data is not available.

Recollimation is a phenomenon of particular importance in the dynamic evolution of jets and in the emission of high-energy radiation. Additionally, the full comprehension of this phenomenon provides insights into fundamental properties of jets in the vicinity of the Active Galactic Nucleus (AGN). Three-dimensional (magneto-)hydrodynamic simulations revealed that the jet conditions at recollimation favor the growth of strong instabilities, challenging the traditional view-supported from two-dimensional simulations-of confined jets undergoing a series of recollimation and reflection shocks. To investigate the stability of relativistic jets in AGNs at recollimation sites, we perform a set of long duration three-dimensional relativistic hydrodynamic simulations with the state-of-the-art PLUTO code, to focus on the development of hydrodynamical instabilities. We explore the non-linear growth of the instabilities and their effects on the physical jet properties as a function of the initial jet parameters: jet Lorentz factor, temperature, opening angle and jet-environment density-contrast. The parameter space is designed to describe low-power, weakly magnetized jets at small distances from the core (around the parsec scale). All collimating jets we simulated develop instabilities. Recollimation instabilities decelerate the jet, heat it, entrain external material, and move the recollimation point to shorter distances from the core. This is true for both conical and cylindrical jets. The instabilities, that are first triggered by the centrifugal instability, appear to be less disruptive in the case of narrower, denser, more relativistic, and warmer jets. These results provide valuable insights into the complex processes governing AGN jets and could be used to model the properties of low-power, weakly magnetized jetted AGNs.

Context: Fast radio bursts (FRBs) are bright millisecond radio events of unknown extragalactic origin. Magnetars are among the main contenders. Some sources, the repeaters, produce multiple events but so far generally without the characteristic periodicity that one could associate with the spin of a neutron star. Aims: Assuming that the bursts originate from a magnetar magnetosphere, we aim to fit our geometrical model to the two main repeaters of the CHIME/FRB catalogue, namely FRB 20180814A and FRB 20180916B, and thus characterise the star. Methods: The model can generate dynamic spectra that can be directly compared to FRBs. We applied nested sampling in order to evaluate the main parameters of the model. These parameters being common to all bursts from a given repeater, they were fitted together as a single dataset. Results: We constrained the spin and magnetic parameters of the star, which were encoded into burst spectro-temporal morphologies. We estimate that a very strong toroidal magnetic component together with spin periods of, respectively, $2.3\_{-0.5}^{+0.5} ~ \rm s$ and $0.8\_{-0.2}^{+0.1} ~ \rm s$ best explain the data. We argue that this points towards young magnetars with super-twisted magnetospheres, and possibly low-field magnetars.

We discuss an interacting dark sector model featuring decaying vacuum energy and dark matter empowered by gravitationally induced matter creation. Motivated by quantum field theoretic considerations of vacuum decay and adiabatic particle production, we analyse both the background dynamics and the growth rate of perturbations. The model is confronted with diverse datasets, including Cosmic Chronometers, Pantheon Type Ia Supernovae, Baryon Acoustic Oscillations, Cosmic Microwave Background distance priors, weighted linear growth rate measurements and an $H_0$ prior, with parameter estimation performed via Markov Chain Monte Carlo (MCMC) methods. Model comparison is carried out using the Akaike and Deviance Information Criterion. Our results show a consistent transition from a decelerated to an accelerated expansion phase, with present Hubble parameter estimates lying between the Planck and SH0ES values, thereby easing the Hubble tension. The structure growth parameter $S_8$ is also compatible with Planck 2018 and recent weak lensing surveys. A thermodynamic analysis confirms consistency with the generalized second law, and including Casimir contributions provides further insights into the model's dynamics. Overall, the proposed model effectively captures the Universe's evolution at both theoretical and observational levels.}

We performed a comprehensive analysis of flux and color variability in a redshift-matched sample of Seyfert galaxies, comprising 23 gamma-ray-detected narrow-line Seyfert 1 galaxies (gNLS1s), 190 non-gamma-ray-detected narrow-line Seyfert 1 galaxies (ngNLS1s), and 10 gamma-ray-detected broad-line Seyfert 1 galaxies (gBLS1s). Utilizing multi-band light curves from the Zwicky Transient Facility (ZTF) in g, r, and i bands, along with mid-infrared (MIR) observations in W1 and W2 bands from the Wide-Field Infrared Survey Explorer (WISE), we observed that gBLS1s exhibit more significant variability than gNLS1s, while ngNLS1s display minimal variability across both optical and MIR wavelengths. The pronounced variability in gBLS1s may be attributed to a more closely aligned jet relative to the observer's line of sight or their comparatively lower accretion rates. In contrast, the subdued variability in ngNLS1s suggests that their flux changes are primarily driven by accretion disk instabilities. A strong correlation between optical and MIR variability amplitudes across different time scales supports the reprocessing scenario, where accretion disk emission variations are re-emitted by surrounding dust. Furthermore, our long-term color variability analysis revealed both stronger bluer-when-brighter (BWB) and redder-when-brighter (RWB) trends from the current sample, but a stronger RWB in approximately 50%, 49%, and 50% of gNLS1s, ngNLS1s, and gBLS1s, respectively, in the longer side of the optical wavelength, and 55%, 28%, and 30% in the MIR wavelength, strengthen the reprocessing scenario. The prevalent RWB trend observed in both optical and MIR wavelengths from the current sample on the longer time scales is likely associated with accretion disk instabilities.

We propose a new method that reveal the nature of dark energy (DE) evolution. Specifically, the method consists of studying the evolving trend regarding the effective running Hubble constant: when it increases, it indicates a quintessence nature, and when it decreases, it reveals a phantom behavior. Within the framework of the dark energy models we analyze three parameterizations: the $w$CDM model, a reduced Chevallier-Polarski-Linder (CPL) model and a new theoretical model based on the possible creation of dark energy by the time-varying gravitational field of the expanding Universe. For each DE model, we construct a theoretical effective running Hubble constant, i.e. a function of the redshift, which highlights the difference between modified dynamics and the $\Lambda$CDM-one. Furthermore, these dark energy models are compared to the phenomenological model of a decreasing trend of the Hubble constant as a function of the redshift, called the power-law model (PL) and the $\Lambda$CDM one. These three theoretical functions are fitted against the binned SNe Ia data samples, i.e. the Pantheon and the Master samples, the latter being a collection of SNe Ia from 4 catalogs: Dark Energy Survey (DES), PantheonPlus, Pantheon and Joint Lightcurve Analysis (JLA), without duplicated SNe Ia, called the Master sample. The main result of our study is that the phenomenological PL model is statistically favored compared to the other proposed scenarios, both for the Pantheon and the Master samples. At this stage, the SNe Ia data do not indicate that the evolution of dark energy models among the studied ones is favored respect to the $\Lambda$CDM. Nevertheless, the binned Pantheon sample allows for a discrimination of the nature of dark energy at least at the $1\,\sigma$ level via the fit of the effective running Hubble constant.

Cosmological constraints derived from weak lensing (WL) surveys are limited by baryonic effects, which suppress the non-linear matter power spectrum on small scales. By combining WL measurements with data from external tracers of the gas around massive structures, it is possible to calibrate baryonic effects and, therefore, obtain more precise cosmological constraints. In this study, we generate mock data for a Stage-IV weak lensing survey such as the Legacy Survey of Space and Time (LSST), X-ray gas fractions, and stacked kinetic Sunyaev-Zel'dovich (kSZ) measurements, to jointly constrain cosmological and astrophysical parameters describing baryonic effects (using the Baryon Correction Model - BCM). First, using WL data alone, we quantify the level to which the BCM parameters will need to be constrained to recover the cosmological constraints obtained under the assumption of perfect knowledge of baryonic feedback. We identify the most relevant baryonic parameters and determine that they must be calibrated to a precision of $\sim 10$-$20\%$ to avoid significant degradation of the fiducial WL constraints. We forecast that long-term X-ray data from $\sim 5000$ clusters should be able to reach this threshold for the parameters that characterise the abundance of hot virialised gas. Constraining the distribution of ejected gas presents a greater challenge, however, but we forecast that long-term kSZ data from a CMB-S4-like experiment should achieve the level of precision required for full self-calibration.

The magnetic fields and dynamical processes in the solar polar regions play a crucial role in the solar magnetic cycle and in supplying mass and energy to the fast solar wind, ultimately being vital in controlling solar activities and driving space weather. Despite numerous efforts to explore these regions, to date no imaging observations of the Sun's poles have been achieved from vantage points out of the ecliptic plane, leaving their behavior and evolution poorly understood. This observation gap has left three top-level scientific questions unanswered, 1) How does the solar dynamo work and drive the solar magnetic cycle? 2) What drives the fast solar wind? 3) How do space weather processes globally originate from the Sun and propagate throughout the solar system? The Solar Polar-orbit Observatory (SPO) mission, a solar polar exploration spacecraft, is proposed to address these three unanswered scientific questions by imaging the Sun's poles from high heliolatitudes. In order to achieve its scientific goals, SPO will carry six remote-sensing and four in-situ instruments to measure the vector magnetic fields and Doppler velocity fields in the photosphere, to observed the Sun in the extreme ultraviolet, X-ray, and radio wavelengths, to image the corona and the heliosphere up to 45 $R_\odot$, and to perform in-situ detection of magnetic fields, and low- and high-energy particles in the solar wind.

The newly discovered Galactic transient MAXI J1744-294 went into its first X-ray outburst in 2025. We study the spectral properties of this source in the 2-10 keV energy band during this outburst using X-ray data from the XRISM satellite for both of its Resolve and Xtend instruments, taken on March 03, 2025. High-resolution spectroscopy has revealed, for the first time, complex iron line features in this source, corresponding to distinct components of Fe XXV emission and Fe XXVI absorption lines. Such a detailed structure has not been reported in other low-mass X-ray binaries to date, prior to the XRISM era. Our analysis shows that the line complexes arise from two highly ionized plasmas with ionization rate   1000 erg-cm/s with distinct turbulent velocities: one broad ( 2513 km/s) from hot gas at the inner accretion disk and one narrow ( 153 km/s) scattered by nearby photoionized gas. These results offer new insight into the reprocessing of continuum in stratified media, either in the accretion disk or winds, or both, for XRBs in the soft state. The data are well described by models with spin, mass of the black hole, and accretion disk inclination 0.63-0.70, 5.7-10.1 Solar masses, and 19-24 degrees. The fitted spectral model parameters suggest that the source is in the soft spectral state. The source is situated in a crowded field near the Galactic center, resulting in a large hydrogen column density.

The nuclear rates for reactions involving 12C and 16O are key to compute the energy release and nucleosynthesis of massive stars during their evolution. These rates shape the stellar structure and evolution, and impact the nature of the final compact remnant. We explore the impact of new nuclear reaction rates for 12C({\alpha},{\gamma})16O, 12C+12C, 12C+16O and 16O+16O reactions for massive stars. We aim to investigate how the structure and nucleosynthesis evolve and how these processes influence the stellar fate. We computed stellar models using the GENEC code, including updated rates for 12C({\alpha},{\gamma})16O and, for the three fusion reactions, new rates following a fusion suppression scenario and new theoretical rates obtained with TDHF calculations. The updated 12C({\alpha},{\gamma})16O rates mainly impact the chemical structure evolution changing the 12C/16O ratio with little effect on the CO core mass. This variation in the 12C/16O ratio is critical for predicting the stellar fate, which is very sensitive to 12C abundance. The combined new rates for 12C+12C and 16O+16O fusion reactions according to the HIN(RES) model lead to shorter C- and O-burning lifetimes, and shift the ignition conditions to higher temperatures and densities. Theoretical TDHF rates primarily affect C-burning, increasing its duration and lowering the ignition temperature. These changes alter the core chemical structure, the carbon shell size and duration, and hence the compactness. They also affect nucleosynthesis. This work shows that accurate reaction rates for key processes in massive star evolution drive significant changes in stellar burning lifetimes, chemical evolution, and stellar fate. In addition, discrepancies between experimental and theoretical rates introduce uncertainties in model predictions, influencing both the internal structure and the supernova ejecta composition.

Gamma-ray bursts (GRBs) offer a powerful window to probe the progenitor systems responsible for the formation of heavy elements through the rapid neutron capture (r-) process, thanks to their exceptional luminosity, which allows them to be observed across vast cosmic distances. GRB 241105A, observed at a redshift of z = 2.681, features a short initial spike (1.5 s) and a prolonged weak emission lasting about 64 s, positioning it as a candidate for a compact binary merger and potentially marking it as the most distant merger-driven GRB observed to date. However, the emerging ambiguity in GRB classification necessitates further investigation into the burst's true nature. Prompt emission analyses, such as hardness ratio, spectral lag, and minimum variability timescales, yield mixed classifications, while machine learning-based clustering places GRB 241105A near both long-duration mergers and collapsar GRBs. We conducted observations using the James Webb Space Telescope (JWST) to search for a potential supernova counterpart. Although no conclusive evidence was found for a supernova, the host galaxy's properties derived from the JWST observations suggest active star formation with low metallicity, and a sub-kpc offset of the afterglow from the host, which appears broadly consistent with a collapsar origin. Nevertheless, a compact binary merger origin cannot be ruled out, as the burst may plausibly arise from a fast progenitor channel. This would have important implications for heavy element enrichment in the early Universe.

Heartbeat stars (HBSs) are ideal laboratories for studying the formation and evolution of binary stars in eccentric orbits and their mutual tidal interactions. We present 42 new HBSs discovered based on TESS-SPOC and QLP data. Their physical parameters have been obtained through modeling with appropriate models. Subsequently, Tidally excited oscillations (TEOs) are detected in ten systems, and their pulsation phases and modes are identified. Most pulsation phases can be explained by the dominant being spherical harmonic degree $l=2$ and azimuthal order $m=0$ or $\pm2$. For TIC 156846634, the harmonic with large deviation ($>3\sigma$) from the expected adiabatic phase can be expected to be a traveling wave or significantly nonadiabatic. The harmonic numbers $n$ = 16 in TIC 184413651 may not be considered as a TEO candidate due to its large deviation ($>2\sigma$) from the adiabatic expectation. Moreover, TIC 92828790 shows no TEOs but exhibits a significant $\gamma$ Dor-type pulsation. The eccentricity-period ($e-P$) relation also shows a positive correlation between eccentricity and period, as well as the existence of orbital circularization. The Hertzsprung-Russell diagram shows that TESS HBSs have higher temperatures and greater luminosities than Kepler HBSs, possibly due to selection effects. This significantly enhances the detectability of massive HBSs and those containing TEOs.

The identification and classification of Fermi blazars are core topics in high-energy astrophysics. To enable precise spatial cross-identification, we constructed two high-precision catalogs: the updated 4FGL-Xiang-DR2 (DR2) and a supplementary version of the fifth edition of Roma-BZCAT (\texttt{5BZCAT\_err}). We then developed and applied a novel four-step analytical pipeline combining cross-matching with the statistical analysis of multi-band flux distributions to identify new Fermi blazars. The analytical pipeline has yielded several key results in the systematic comparison of BZBs and BZQs. We found that among single statistical metrics, kurtosis is the most powerful discriminator (MAD $>$ 1.64). At the overall distribution level, the 1.4 GHz, 843 MHz, 5 GHz, 0.1–2.4 keV, and 0.3–10 keV bands show significant divergence (JSD $>$ 0.3). Building on these findings, our proposed “Box-Cox$+$TND” model successfully fits the observed flux distributions between BZBs and BZQs. Applying this entire pipeline, we successfully identified 17 new blazars. The validity of these associations is strongly supported by our multi-wavelength flux model, which confirms that 15 of the 17 candidates are statistically consistent with the known blazar population, falling within the $2\sigma$ confidence interval. Although the two remaining sources exhibit some statistical deviation in the gamma-ray band, their strong consistency in other wavebands, coupled with high spatial association probabilities, leads us to conclude that their associations are also reliable and should not be readily excluded.

Recent measurements made by the Alpha Magnetic Spectrometer (AMS) have detected accurate positron flux for energy range 1-1000 GeV. The energy spectrum can be best described by two source terms: the low-energy background diffusion term and an unknown high-energy source term. In this article, we discuss the possibility of the emission of positrons originating from dark matter annihilation in two nearby black hole X-ray binaries A0620-00 and XTE J1118+480. We show that the dark matter density spikes around these two black holes can best produce the observed AMS-02 high-energy positron flux due to dark matter annihilation with rest mass $m_{\rm DM} \approx 8000$ GeV via the $W^+W^-$ annihilation channel. This initiates a new proposal to account for the unknown high-energy source term in the AMS-02 positron spectrum.

We present a comprehensive analysis of super-Eddington black hole accretion simulations that solve the GRMHD equations coupled with angle-discretized radiation transport. The simulations span a range of accretion rates, two black hole spins, and two magnetic field topologies, and include resolution studies as well as comparisons with non-radiative models. Super-Eddington accretion flows consistently develop geometrically thick disks supported by radiation pressure, regardless of magnetic field configuration. Radiation generated in the inner disk drives substantial outflows, forming conical funnel regions that limit photon escape and result in very low radiation efficiency. The accretion flows are highly turbulent with thermal energy transport dominated by radiation advection rather than diffusion. Angular momentum is primarily carried outward by Maxwell stress, with turbulent Reynolds stress playing a subdominant role. Both strong and weak jets are produced. Strong jets arise from sufficient net vertical magnetic flux and rapid black hole spin and can effectively evacuate the funnel, enabling radiation to escape through strong geometric beaming. In contrast, weak jets fail to clear the funnel, which becomes obscured by radiation-driven outflows and leads to distinct observational signatures. Spiral structures are observed in the plunging region, behaving like density waves. These super-Eddington models are applicable to a variety of astronomical systems, including ultraluminous X-ray sources, little red dots, and black hole transients.

We select galaxy cluster candidates from the high-redshift (BEST_Z > 0.9) end of the first SRG/eROSITA All-Sky Survey (eRASS1) galaxy cluster catalogue, for which we obtain moderately deep J-band imaging data with the OMEGA2000 camera at the 3.5m telescope of the Calar Alto Observatory. We include J-band data of four additional targets obtained with the three-channel camera at the 2m Fraunhofer telescope at the Wendelstein Observatory. We complement the new J-band photometric catalogue with forced photometry in the i- and z-bands of the tenth data release of the Legacy Survey (LSDR10) to derive the radial colour distribution around the eRASS1 clusters. Without assuming a priori to find a cluster red sequence at a specific colour, we try to find a radially weighted colour over-density to confirm the presence of high-redshift optical counterparts for the X-ray emission. We compare our confirmation with optical properties derived in earlier works based on LSDR10 data to refine the existing high-redshift cluster confirmation of eROSITA-selected clusters. We attempt to calibrate the colour-redshift-relation including the new J-band data by comparing our obtained photometric redshift estimate with the spectroscopic redshift of a confirmed, optically selected, high-redshift galaxy cluster. We confirm 9 out of 18 of the selected galaxy cluster candidates with a radial over-density of similar coloured galaxies for which we provide a photometric redshift estimate. We can report an increase in the relative colour measurement precision from 8% to 4% when including J-band data. In conclusion, our findings indicate a not insignificant spurious contaminant fraction at the high-redshift end (BEST_Z > 0.9) of the eROSITA/eRASS1 galaxy cluster catalogue, as well as it underlines the necessity for wide and deep near infrared imaging data for confirmation and characterisation of high-$z$ galaxy clusters.

The solar wind originates from regions of open magnetic fields on the Sun, but the relevant processes remain unsolved. We present a self-consistent numerical model of the source region of the wind, in which jets similar to those observed on the Sun naturally emerge due to magnetic reconnection between closed and open magnetic fields. In this process material is transferred from closed to open field lines and fed into the solar wind. We quantify the mass flux through the magnetic field connected to the heliosphere and find that it greatly exceeds the amount required to sustain the wind. This supports a decades-old suspicion based on spectroscopic observations and shows that magnetic reconnection in the low solar atmosphere could sustain the solar wind.

We investigate the tidal resonance of the fundamental ($f$-)mode in spinning neutron stars, robustly tracing the onset of the excitation to its saturation, using numerical relativity for the first time. We performed long-term ($\approx15$ orbits) fully relativistic simulations of a merger of two highly and retrogradely spinning neutron stars. The resonance window of the $f$-mode is extended by self-interaction, and the nonlinear resonance continues up to the final plunging phase. We observe that the quasi-circular orbit is maintained throughout since the dissipation of orbit motion due to the resonance is coherent with that due to gravitational waves. The $f$-mode resonance causes a variation in the stellar spin of $\gtrsim6.3\%$ in the linear regime and much more as $\sim33\%$ during the later nonlinear regime. At the merger, a phase shift of $\lesssim40$ radians is rendered in the gravitational waveform as a consequence of the angular momentum and energy transfers into the neutron star oscillations.

Cosmological stasis is a new type of epoch in the cosmological timeline during which the cosmological abundances of different energy components – such as vacuum energy, matter, and radiation – remain constant despite the expansion of the universe. Previous studies have shown that stasis naturally arises in various scenarios beyond the Standard Model, either through sequential decays of states in large towers or via the annihilation of a single particle species in thermal equilibrium with itself. In this work, we demonstrate that stasis can also emerge from the decay of a single particle species whose decay width is dynamically regulated by a scalar field rolling down a Hubble-mass potential. By analyzing the fixed points of the dynamical system, we identify regions of the parameter space where stasis occurs as a global attractor of cosmic evolution. We also find that, depending on the specific abundance configuration, stasis solutions can manifest as either a stable node with asymptotic behavior or a stable spiral exhibiting intrinsic oscillations. Furthermore, we present an explicit model for this realization of stasis and explore its phenomenological constraints and implications.

The lack of a dynamical framework within doubly special relativity theories has impeded the development of a corresponding phenomenology of modified interactions. In this work we show that in a model based on the classical basis of $\kappa$-Poincaré and total momentum conservation, one has a well-defined cross section of the photon-photon annihilation process, once a prescription for the channel treatment is set. The modification of the interaction can lead to observable effects in the opacity of the Universe to very high-energy gamma rays when the gamma-ray energy approaches the energy scale of the deformation. The magnitude and observability of this deformation are examined as functions of the gamma-ray energy and source distance.

We investigate the observational features of exact vacuum solutions in Brans-Dicke (BD) gravity, focusing on their implications for black hole shadow imaging. Motivated by the Event Horizon Telescope (EHT) observations, we revisit a class of BD solutions that exhibit a naked singularity. These solutions, despite lacking a conventional event horizon, exhibit photon spheres and produce shadow-like features. We analyze null geodesics and perform ray-tracing simulations under a simplified, optically thin accretion disk model to generate synthetic images. Our results show that BD naked singularities can cast shadows smaller than those of Schwarzschild black holes of equivalent mass. We identify the parameter space $-3/2 < \omega < 0$ as physically viable, ensuring attractive gravity and the absence of ghost fields. These findings suggest that BD naked singularities are possible candidates for compact astrophysical objects.

We study black hole imaging in the context of geometrically thick accretion disks in Schwarzschild spacetime. By decomposing the emitting region into a set of one-dimensional luminous segments, each characterized by its inclination angle and inner radius, we construct transfer functions that capture key image features-namely, the direct image, lensing ring, and photon ring. This approach allows a unified treatment of disk geometry and viewing angle. We explore three regimes: optically thin, optically thick, and partially optically thick disks. For optically thin flows, increasing the disk thickness (characterized by the half-opening angle $\psi_0$) broadens the lensing ring, gradually bridging the photon ring and the direct image. The photon ring remains narrow, but its position robustly defines the innermost edge of the lensing structure. In the optically thick case, image features are primarily determined by the first intersection of traced light rays with the disk, and we provide analytical criteria for the presence of lensing and photon rings based on the critical deflection angles. For partially optically thick disks, we adopt a simplified radiative transport model and find a critical absorption coefficient $\chi \sim (6M\psi_0)^{-1}$ beyond which the image rapidly transitions from an optically thin- to thick-disk appearance. These results help clarify the respective roles of the photon and lensing rings across different disk configurations, and may offer a useful framework for interpreting future high-resolution black hole observations.

As space exploration extends into cislunar space and further towards Mars, understanding the relativistic effects on clocks on Mars, particularly in relation to multibody gravitational influences, becomes increasingly important for accurate clock synchronization. This study estimates clock rates on Mars and compares them to those on the Moon and Earth. We find that, on average, clocks on Mars tick faster than those on the Earth's geoid by 477 microseconds per day, with a variation of 226 microseconds per day over a Martian year. Additionally, there is an amplitude modulation of approximately 40 microseconds per day over seven synodic cycles. We also introduce a formalism for addressing the effects of solar tides on the Earth-Moon system for predicting clock rates on the Moon and Mars more accurately when compared to using only Keplerian orbit approximations. Our analysis quantifies the relativistic proper time offsets among Martian, lunar, and terrestrial clocks, highlighting important implications for mission planning and the implementation of timekeeping systems on Mars.

We investigate interacting dark energy (IDE) models with phenomenological, non-linear interaction kernels $Q$, specifically $Q_{1}=3H\delta \left(\frac{\rho_{\rm dm}\rho_{\rm de}}{\rho_{\rm dm}+\rho_{\rm de}}\right)$, $Q_{2}=3H\delta \left(\frac{\rho_{\rm dm}^2}{\rho_{\rm dm}+\rho_{\rm de}}\right)$, and $Q_{3}=3H\delta \left(\frac{\rho_{\rm de}^2}{\rho_{\rm dm}+\rho_{\rm de}}\right)$. Using dynamical system techniques developed in our companion paper on linear kernels, we derive new conditions that ensure positive and well-defined energy densities, as well as criteria to avoid future big rip singularities. We find that for $Q_{1}$, all densities remain positive, while for $Q_{2}$ and $Q_{3}$ negative values of either DM or DE are unavoidable if energy flows from DM to DE. We also show that for $Q_{1}$ and $Q_{2}$ a big rip singularity always arises in the phantom regime $w<-1$, whereas for $Q_{3}$ this fate may be avoided if energy flows from DE to DM. In addition, we provide new exact analytical solutions for $\rho_{\rm dm}$ and $\rho_{\rm de}$ in the cases of $Q_{2}$ and $Q_{3}$, and obtain new expressions for the effective equations of state of DM, DE, the total fluid, and the reconstructed dynamical DE equation of state ($w_{\rm dm}^{\rm eff}$, $w_{\rm de}^{\rm eff}$, $w_{\rm tot}^{\rm eff}$, and $\tilde{w}$). Using these results, we discuss phantom crossings, evaluate how each kernel addresses the coincidence problem, and apply statefinder diagnostics to compare the models. These findings extend the theoretical understanding of non-linear IDE models and provide analytical tools for future observational constraints.

Interacting dark energy (IDE) models, in which dark matter (DM) and dark energy (DE) exchange energy through a non-gravitational interaction, have long been proposed as candidates to address key challenges in modern cosmology. These include the coincidence problem, the $H_0$ and $S_8$ tensions, and, more recently, the hints of dynamical dark energy reported by the DESI collaboration. Given the renewed interest in IDE models, it is crucial to fully understand their parameter space when constraining them observationally, especially with regard to the often-neglected issues of negative energy densities and future big rip singularities. In this work, we present a comparative study of the general linear interaction $Q=3H(\delta_{\rm dm}\rho_{\rm dm} + \delta_{\rm de}\rho_{\rm de})$ and four special cases: $Q=3H\delta(\rho_{\rm dm}+\rho_{\rm de})$, $Q=3H\delta(\rho_{\rm dm}-\rho_{\rm de})$, $Q=3H\delta \rho_{\rm dm}$, and $Q=3H\delta \rho_{\rm de}$. For these five models, we perform a dynamical system analysis and derive new conditions that ensure positive, real, and well-defined energy densities throughout cosmic evolution, as well as criteria to avoid future big rip singularities. We obtain exact analytical solutions for $\rho_{\rm{dm}}$, $\rho_{\rm{de}}$, the effective equations of state ($w_{\mathrm{eff}}^{\rm{dm}}$, $w_{\mathrm{eff}}^{\rm{de}}$, $w_{\mathrm{eff}}^{\rm{tot}}$), and a reconstructed dynamical DE equation of state $\tilde{w}$. Using these results, we examine phantom crossings, address the coincidence problem, and apply the statefinder diagnostic to distinguish between models. We show that energy transfer from DM to DE inevitably produces negative energy densities and make future singularities more likely, while transfer from DE to DM avoids these pathologies and is thus theoretically favored.

We present an overview of the main results from our two companion papers that are relevant for observational constraints on interacting dark energy (IDE) models. We provide analytical solutions for the dark matter and dark energy densities, $\rho_{\rm dm}$ and $\rho_{\rm de}$, as well as the normalized Hubble function $h(z)$, for eight IDE models. These include five linear IDE models, namely $Q=3H(\delta_{\rm dm} \rho_{\rm dm} + \delta_{\rm de} \rho_{\rm de})$ and four special cases: $Q=3H\delta(\rho_{\rm dm}+\rho_{\rm de})$, $Q=3H\delta(\rho_{\rm dm}-\rho_{\rm de})$, $Q=3H\delta \rho_{\rm dm}$, and $Q=3H\delta \rho_{\rm de}$, together with three non-linear IDE models: $Q=3H\delta \left( \tfrac{\rho_{\rm dm} \rho_{\rm de}}{\rho_{\rm dm}+\rho_{\rm de}} \right)$, $Q=3H\delta \left( \tfrac{\rho_{\rm dm}^2}{\rho_{\rm dm}+\rho_{\rm de}} \right)$, and $Q=3H\delta \left( \tfrac{\rho_{\rm de}^2}{\rho_{\rm dm}+\rho_{\rm de}} \right)$. For these eight models, we present conditions to avoid imaginary, undefined, and negative energy densities. In seven of the eight cases, negative densities arise if energy flows from DM to DE, implying a strong theoretical preference for energy transfer from DE to DM. We also provide conditions to avoid future big rip singularities and evaluate how each model addresses the coincidence problem in both the past and the future. Finally, we propose a set of approaches and simplifying assumptions that can be used when constraining IDE models, by defining regimes that restrict the parameter space according to the behavior researchers are willing to tolerate.