In this work, we report, for the first time, a quasi-periodic oscillation (QPO) in the $\gamma$-ray band of 4FGL J0309.9-6058, also known as PKS 0308-611. We employed three analytical methods (the Lomb-Scargle periodogram, REDFIT, and the weighted wavelet Z-transform) to analyze the QPO signal using \textit{Fermi} $\gamma$-ray light curve data. The analysis reveals a potential QPO during MJD 57983$-$60503, with a period of approximately 550 days and a maximum local significance of 3.72$\sigma$ and global significance of 2.72$\sigma$ derived from the WWZ analysis. To validate this result, we applied Gaussian Process (GP) to the same light curve, which independently confirms the presence of QPO signal consistent with our Fourier-based results. We further extended the analysis to the full duration of the \textit{Fermi} observations, and the results consistently support and strengthen the presence of this QPO signal. Additionally, a time lag between the optical and $\gamma$-ray bands indicates separate emission regions for these two bands. Given the year-like timescale of the QPO signal and the fact that a QPO signal with local significance over 3$\sigma$ for full \textit{Fermi}-LAT observed time, we suggest that the QPO is most likely caused by a precessing jet.

Accretion in black hole X-ray binaries is commonly believed to be supplied by the Roche lobe overflow or the stellar wind. The former is thought to form a geometrically thin disc while the diffuse wind could form a geometrically thick hot accretion flow. In this paper, we instead consider a more generalised case, i.e., accretion with both cold and hot gas supplies, which feed a disc and a corona respectively. We investigate the interaction of disc and corona by analysing the energy coupling and matter exchange, i.e. corona condensation/disc evaporation, with a semi-analytical method. It is found that the accretion geometry in the radial direction and the resultant emission spectrum depend strongly on both the total gas supply rate and the ratio of cold and hot gases. For gas supply rates of a few percent of the Eddington value, diverse geometries and spectral shapes are possible, depending on the fraction of cold gas supply. This provides an interpretation for the various spectra observed in intermediate states. However, at higher accretion rates, regardless of the form of the feeding gas, the inner accretion flow is always disc-dominated, implying an inevitable transition to the soft state, while at very low gas supply rates, hard state spectrum dominated by the hot flow is expected. We also present the predicted hardness-intensity correlation of Cygnus X-1, and constrain the value of the viscosity parameter of the accretion flow to the range of 0.25--0.35 by comparing our results with MAXI observations.

Accurate measurements of cosmic-ray fragmentation cross sections are essential for maximizing the physics potential of precise measurements of secondary and primary cosmic-ray fluxes from current balloon and space-borne experiments. NA61/SHINE, operating at the CERN SPS H2 beamline, is uniquely suited to studying these interactions at energies above 10 GeV/c per nucleon. In this contribution, we present the fragmentation cross sections for the breakup of carbon into $^{10}$B, $^{11}$B and $^{11}$C at 13.5 GeV/c per nucleon that are needed for interpreting the cosmic-ray boron-to-carbon ratio. These results are based on data from a pilot run conducted in 2018. We also give an overview of the high-statistics data-taking campaign in 2024, which covered projectile nuclei from lithium to silicon. With over 40 million recorded beam triggers, this data set will enable the reconstruction of the full reaction network required to study light secondary cosmic rays. Furthermore, we report on data collected in 2025 with a primary oxygen beam at 150 GeV/c per nucleon, aimed at verifying the expected flattening of fragmentation cross sections at high energies.

Wideband timing of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) datasets, where a single time-of-arrival (TOA) and a single dispersion measure (DM) are measured using the entire bandwidth of each observation, was first done for the 12.5-year dataset, and proved to be invaluable for characterizing the time-varying dispersion measure, reducing the data volume, and for improving the overall timing precision. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) Telescope has been observing most NANOGrav millisecond pulsars (MSPs) at nearly daily cadence (compared to roughly monthly cadence for other NANOGrav observations) since 2019 with the objective of integration into future pulsar timing array (PTA) datasets. In this paper, we show the results of integration of high-cadence, low-observing-frequency CHIME data with data from the NANOGrav experiment for an isolated MSP PSR~J0645$+$5158 and three binary MSPs PSR~J1012$+$5307, PSR~J2145$-$0750, and PSR~J2302$+$4442. Using a wideband timing pipeline which we also describe, we present updated timing results for all four sources, including improvements in measurements of relativistic post-Keplerian parameters for the three binary pulsars in this analysis. For PSR~J2302$+$4442, we report an updated strong detection of Shapiro delay from which we measured a companion mass of $0.35^{+0.05}_{-0.04}\ M_{\odot}$, a pulsar mass of $1.8^{+0.3}_{-0.3}\ M_{\odot}$, and an orbital inclination of ${80^{\circ}}^{+1}_{-2}$. We also report updated constraints on the reflex motion for PSR~J2145$-$0750 using a combination of Very Long Baseline Array astrometry and our updated measurement of the time derivative of the projected semi-major axis of the pulsar orbit as a prior.

Kilonovae, the ultraviolet/optical/infrared counterparts to binary neutron star mergers, are an exceptionally rare class of transients. Optical follow-up campaigns are plagued by impostors whose early evolution masquerades as the rapid radioactive decay of heavy elements. In this work, we present an analysis of the multi-wavelength dataset of supernova (SN) 2025ulz, a proposed kilonova candidate following the low-significance detection of gravitational waves originating from the potential binary neutron star merger S250818k. Despite an early rapid decline in brightness, our multi-wavelength observations of SN 2025ulz reveal that it is a type IIb supernova. As part of this analysis, we demonstrate the capabilities of a novel quantitative scoring algorithm to determine the likelihood that a transient candidate is a kilonova, based primarily on its 3D location and light curve evolution. We also apply our scoring algorithm to other transient candidates in the localization volume of S250818k and find that, at all times after the discovery of SN 2025ulz, there are $\geq 4$ candidates with a score more promising than SN 2025ulz. During future kilonova searches, this type of scoring algorithm will be useful to rule out contaminating transients in real time, optimizing the use of valuable telescope resources.

Some compact stars may contain deconfined quark matter, forming hybrid stars or quark stars. If the quark matter forms an inhomogeneous condensate in the crystalline color superconducting phase, its rigidity may be high enough to noticeably alter the stellar properties. In this paper, we investigate whether these elastic stars follow the universal relations, i.e., relations insensitive to equations of state, that have been well established for fluid stars. We improve upon previous studies by allowing quark matter in the background, static, and spherically symmetric configuration to be sheared. Such background shear can be treated in the form of an effective pressure anisotropy. We then calculate the moment of inertia $I$, tidal deformability ${\lambda}_2$, and spin-induced quadrupole moment $Q$ of these models with pressure anisotropy. The $I$-${\lambda}_2$-$Q$ universal relations for the elastic hybrid (quark) star models are valid up to a variation of $\approx2\,(3)\%$, larger than that for typical fluid star models, when the maximal magnitude of quark matter shear modulus is considered in the crystalline color superconducting phase from realistic calculations. The uncertainty in universal relations related to the stellar compactness for these elastic star models, on the other hand, remain comparable to those for typical fluid star models. Our results demonstrate the validity of universal relations for hybrid stars and quark stars with a realistic degree of pressure anisotropy due to the crystalline color superconducting quark matter.

The LIGO, Virgo, and KAGRA (LVK) gravitational-wave observatories have opened new scientific research in astrophysics, fundamental physics, and cosmology. The collaborations that build and operate these observatories release the interferometric strain data as well as a catalogue of observed signals with accompanying Bayesian posterior distributions. These posteriors, in the form of equally-weighted samples, form a dataset that is used by a multitude of further analyses seeking to constrain the population of merging black holes, identify lensed pairs of signals, and much more. However, many of these analyses rely, often implicitly, on the ability to reconstruct the likelihood and prior from the inputs to the analysis and apply resampling (a statistical technique to generate new samples varying the underlying analysis assumptions). In this work, we first provide a guide on how to reconstruct and modify the posterior density accurately from the inputs for analyses performed with the Bilby inference library. We then demonstrate and compare resampling techniques to produce new posterior sample sets and discuss Pareto-smoothing to improve the efficiency. Finally, we provide examples of how to use resampling to study observed gravitational-wave signals. We hope this guide provides a useful resource for those wishing to use open data products from the LVK for gravitational-wave astronomy.

Rapidly rotating neutron stars (NSs) are promising targets for continuous gravitational-wave (CGW) searches with current and next-generation ground-based GW detectors. In this Letter, we present the first study of thermal deformations in super-Eddington magnetized NSs with column accretion, where magnetic fields induce anisotropic heat conduction that leads to crustal temperature asymmetries. We compute the resulting mass quadrupole moments and estimate the associated CGW strain amplitudes. Our results show that Galactic magnetized NSs undergoing super-Eddington column accretion can emit detectable CGWs in upcoming observatories. Assuming a 2-yr coherent integration, the Einstein Telescope and Cosmic Explorer could detect such CGW signals from rapidly spinning NSs with spin periods $P \lesssim 20\,\rm ms$, while the LIGO O5 run may detect systems with $P \lesssim 6 \,{\rm ms}$. These findings suggest that super-Eddington magnetized NSs could represent a new class of CGW sources, providing a unique opportunity to probe the NS crust and bridge accretion physics with GW astronomy.

Kilonovae are the scientifically rich, but observationally elusive, optical transient phenomena associated with compact binary mergers. Only a handful of events have been discovered to date, all through multi-wavelength (gamma ray) and multi-messenger (gravitational wave) signals. Given their scarcity, it is important to maximise the discovery possibility of new kilonova events. To this end, we present our follow-up observations of the gravitational-wave signal, S250818k, a plausible binary neutron star merger at a distance of $237 \pm 62$ Mpc. Pan-STARRS tiled 286 and 318 square degrees (32% and 34% of the 90% sky localisation region) within 3 and 7 days of the GW signal, respectively. ATLAS covered 70% of the skymap within 3 days, but with lower sensitivity. These observations uncovered 47 new transients; however, none were deemed to be linked to S250818k. We undertook an expansive follow-up campaign of AT 2025ulz, the purported counterpart to S250818k. The griz-band lightcurve, combined with our redshift measurement ($z = 0.0849 \pm 0.0003$) all indicate that SN 2025ulz is a SN IIb, and thus not the counterpart to S250818k. We rule out the presence of a AT 2017gfo-like kilonova within $\approx 27$% of the distance posterior sampled by our Pan-STARRS pointings ($\approx 9.1$% across the total 90% three-dimensional sky localisation). We demonstrate that early observations are optimal for probing the distance posterior of the three-dimensional gravitational-wave skymap, and that SN 2025ulz was a plausible kilonova candidate for $\lesssim 5$ days, before ultimately being ruled out.

Recent observations from NICER in X-rays and LIGO/Virgo in gravitational waves have provided critical constraints on the mass, radius, and tidal deformability of neutron stars, imposing stringent limits on the equation of state (EOS) and the behavior of ultra-dense matter. However, several key parameters influencing the EOS, such as the maximum mass of neutron stars, spin-down rates, and the potential role of exotic matter in their cores, remain subject of ongoing debate. Here we present a new approach to constraining the EOS by analyzing the X-ray afterglows of some short gamma-ray bursts, focusing on "the internal plateau" phase and its abrupt decay, which reflect the spin-down and possible collapse of a supra-massive neutron star into a black hole. By linking critical neutron star masses with black hole formation criteria and the observational data from Swift's BAT and XRT instruments with compact object models, we explore three representative EOSs that range from "soft" to "stiff". Our result supports a maximum mass for neutron stars of approximately 2.39 solar masses at the threshold of black hole formation. This conclusion holds under assumptions of magnetar-powered X-ray plateaus, constant radiative efficiency, isotropic emission, and full Kerr black hole energy extraction; deviations could influence the inferred results. Our results demonstrate the critical role of neutron star/black hole physics in probing dense nuclear matter and provide a novel framework for exploring extreme astrophysical environments.

Surrogate modeling of eccentric binary black hole waveforms has remained challenging. The complicated morphology of these waveforms due to the eccentric orbital timescale variations makes it difficult to construct accurate and efficient surrogate models, especially for waveforms long enough to cover the sensitivity band of the current ground-based gravitational wave detectors. We present a novel and scalable surrogate building technique which makes surrogate modeling of long-duration eccentric binary black hole waveforms both feasible and highly efficient. The technique aims to simplify the harmonic content of the intermediate eccentric surrogate data pieces by modeling them in terms of an angular orbital element called the mean anomaly, instead of time. We show that this novel parameterization yields an order of magnitude fewer surrogate basis functions than using the contemporary parameterization in terms of time. We show that variations in surrogate data-pieces across parameter space become much more regular when expressed in terms of the instantaneous waveform eccentricity and mean anomaly, greatly easing their parameter-space fitting. The methods presented in this work make it feasible to build long-duration eccentric surrogates for the current as well as future third-generation gravitational wave detectors.

Cosmic rays (CRs) are an important source of feedback in a variety of astrophysical contexts. Magneto-hydrodynamical (MHD) simulations treating CRs as a fluid have shown that how their feedback operates is strongly dependent on their transport properties such as diffusion and streaming. In this paper we introduce the numerical implementation, in the adaptive-mesh-refinement MHD code RAMSES, of the grey two-moment formulation of CR fluid dynamics, which follows the energy density and its associated three-dimensional flux. This method is tested for CR diffusion, streaming, and advection in a series of multi-dimensional tests including shocks to check the robustness and stability of this numerical two-moment CRMHD solver. We finally use the new two-moment CR implementation in a complex simulation of an isolated galactic disc producing galaxy-wide outflows launched by small-scale supernova explosions, and compare it with a previously existing one-moment formulation in the same code.

The past decade has transformed our ability to observe the Universe. Via gravitational waves, merging black holes and neutron stars can now be directly detected, offering unprecedented opportunities to test General Relativity and explore astrophysics in a new way. Driven by this breakthrough, the next generation of detectors is being developed to observe a wider range of sources with greater precision, ushering in a new era in gravitational-wave astronomy: leveraging black holes as probes of new physics. This thesis investigates how astrophysical environments, such as plasma, dark-matter structures, and clouds of ultralight bosons, affect black holes and their gravitational-wave signatures. After a short overview of gravitational-wave astrophysics, I study three classes of scenarios. (i) Isolated black holes: I examine boson clouds around black holes, their electromagnetic couplings and the role of surrounding plasma. (ii) Ringdown: I show that plasma can strongly modify the ringdown of charged black holes, whereas realistic dark-matter halos produce no detectable deviations even for next-generation detectors. (iii) Inspiral: for extreme-mass-ratio inspirals with boson clouds, I find that orbital resonances typically destroy the cloud unless the orbit is nearly counter-rotating, yielding new and exciting observational signatures. Entering the relativistic regime, I develop a self-consistent perturbative framework to model generic environments in extreme-mass-ratio binaries and apply it to the boson-cloud case. Finally, I construct a model for binaries repeatedly crossing active galactic-nucleus disks and track their long-term orbital evolution. The results of this thesis show how black hole environments shape gravitational-wave signals and open avenues for testing new physics with future observatories such as LISA or the Einstein Telescope.

Neural density estimation has seen widespread applications in the gravitational-wave (GW) data analysis, which enables real-time parameter estimation for compact binary coalescences and enhances rapid inference for subsequent analysis such as population inference. In this work, we explore the application of using the Kolmogorov-Arnold network (KAN) to construct efficient and interpretable neural density estimators for lightweight posterior construction of GW catalogs. By replacing conventional activation functions with learnable splines, KAN achieves superior interpretability, higher accuracy, and greater parameter efficiency on related scientific tasks. Leveraging this feature, we propose a KAN-based neural density estimator, which ingests megabyte-scale GW posterior samples and compresses them into model weights of tens of kilobytes. Subsequently, analytic expressions requiring only several kilobytes can be further distilled from these neural network weights with minimal accuracy trade-off. In practice, GW posterior samples with fidelity can be regenerated rapidly using the model weights or analytic expressions for subsequent analysis. Our lightweight posterior construction strategy is expected to facilitate user-level data storage and transmission, paving a path for efficient analysis of numerous GW events in the next-generation GW detectors.

Using data from Swift-BAT and Fermi-GBM, we report the first detection of a high-confidence quasi-periodic oscillation (QPO) in the thermal emission of gamma-ray burst GRB~240825A. The spectral analysis of the burst reveals two radiation components, including a thermal emission dominant in $100 \text{--} 300\,\mathrm{keV}$ and a non-thermal emission spanning a wide energy range. During the interval $2.07 \text{--} 3.25\,\mathrm{s}$ post-trigger, a strong QPO signal at $6.37\,\mathrm{Hz}$ ($\gtrsim 5\sigma$ confidence) is identified in the $100 \text{--} 300\,\mathrm{keV}$ thermal-dominated band. The variability analysis of the non-thermal component ($15 \text{--} 30\,\mathrm{keV}$) uncovered a $0.67\,\mathrm{Hz}$ QPO, consistent with light-curve modeling using periodic fast-rise exponential decay pulses. The strong QPO in the photospheric emission directly indicates a quasi-periodic oscillating jet. Together with the non-thermal emission variability, we show that this QPO can be explained in terms of a helical structure in the jet, where the viewing angle to the dominant emission region in the jet undergoes periodic changes.

Thermonuclear supernovae (SNe) are the result of the nuclear transformation of carbon/oxygen (C/O) white dwarfs (WDs) to the radioactive element $^{56}\mathrm{Ni}$ and intermediate mass elements (IMEs) like Ca, Ar, etc. Most progenitor scenarios involve a companion star which donates matter to the exploding white dwarf, implying a fundamental prediction: the formation of a wake in the explosive ejecta as it runs into and moves past the companion star. This wake leaves an indelible imprint on the ejecta's density, velocity, and composition structure that remains fixed as the ejecta reaches homologous expansion. We simulate the interaction of the ejecta and Roche-lobe filling donor in a double degenerate double detonation Type Ia progenitor scenario and explore the detectability of this imprint in late-time nebular phase spectroscopy of Type Ia SNe under the assumption of local heating ($t > 200$ days). At these times, the velocity profiles of forbidden emission lines reflect the velocity distribution of all of the ejecta and the critical electron density for that forbidden line. We explicitly calculate line shapes for the [Co III] $11.89 \mu\mathrm{m}$ line that traces the initial $^{56}\mathrm{Ni}$ distribution and the [Ar III] $8.99 \mu\mathrm{m}$ line, which traces a typical intermediate mass element. We predict the viewing angle dependence of the line shape, present a tool to quickly calculate optically thin line shapes for various 3D density-velocity profiles and discuss JWST observations.

The origin of Galactic cosmic rays (CRs), particularly around the knee region ($\sim$3 PeV), remains a major unsolved question. Recent observations by LHAAASO suggest that the knee is shaped mainly by protons, with a transition to heavier elements at higher energies. Microquasars -- compact jet-emitting sources -- have emerged as possible PeV CR accelerators, especially after detections of ultrahigh-energy gamma rays from these systems. We propose that the observed proton spectrum (hard below a few PeV, steep beyond) arises from the reacceleration of sub-TeV Galactic CRs via shear acceleration in large-scale microquasar jet-cocoon structures. Our model also naturally explains the observed spectrum of energies around a few tens of PeV by summing up heavier nuclei contributions. Additionally, similar reacceleration processes in radio galaxies can contribute to ultrahigh-energy CRs, bridging Galactic and extragalactic origins. Combined with low-energy CRs from supernova remnants and galaxy clusters around the second knee region, this scenario could provide a unified explanation for CRs across the entire energy spectrum.

Future mHz gravitational wave (GW) interferometers will precisely probe massive black hole environments, such as accretion discs, cold dark matter overdensities, and clouds of ultralight bosons, as long as we can accurately model the dephasing they induce on the waveform of extreme mass-ratio inspirals (EMRIs). Most existing models rely on extrapolations from Newtonian results to model the interaction of the small black hole in an EMRI system with the environment surrounding the massive black hole. Here, we present a fully relativistic formalism to model such interaction with collisionless environments, focusing on the case of cold dark matter overdensities, like 'spikes' and 'mounds'. We implement our new formalism in the FastEMRIWaveforms framework and show that the resulting waveforms are significantly different from those based on a Newtonian treatment of environmental effects. Our results indicate that a fully relativistic treatment is essential to capture the environmental dephasing of GW signals from EMRIs accurately.