Next-generation GW detectors will produce a high rate of temporally overlapping signals from unrelated compact binary coalescences. Such overlaps can bias parameter estimation (PE) and mimic signatures of other physical effects, such as gravitational lensing. In this work, we investigate how overlapping signals can be degenerate with gravitational lensing by focusing on two scenarios: Type-II strong lensing and microlensing by an isolated point-mass lens. We simulate quasicircular binary black-hole pairs with chirp-mass ratios $\mathscr{M}_{\rm B}/\mathscr{M}_{\rm A}\in\{0.5,\,1,\,2\}$, SNR ratios $\mathrm{SNR}_{\rm B}/\mathrm{SNR}_{\rm A}\in\{0.5,\,1\}$, and coalescence-time offsets $\Delta t_{\rm c}\in[-0.1,\,0.1]~\mathrm{s}$. Bayesian PE and fitting-factor studies show that the Type-II lensing hypothesis is favored over the unlensed quasicircular hypothesis ($\log_{10}\mathscr{B}^{\rm L}_{\rm U}>1$) only in a small region of the overlapping parameter space with $\mathscr{M}_{\rm B}/\mathscr{M}_{\rm A}\gtrsim1$ and $|\Delta t_{\rm c}|\leq0.03~\rm{s}$.. Meanwhile, false evidence for microlensing signatures can arise because, to a reasonable approximation, the model produces two superimposed images whose time delay can closely match $|\Delta t_{\rm c}|$. Overall, the inferred Bayes factor depends on relative chirp-mass ratios, relative loudness, difference in coalescence times, and also the absolute SNRs of the overlapping signals. Cumulatively, our results indicate that overlapping black-hole binaries with nearly equal chirp masses and comparable loudness are likely to be falsely identified as lensed. Such misidentifications are expected to become more common as detector sensitivities improve. While our study focuses on ground-based detectors using appropriate detectability thresholds, the findings naturally extend to next-generation GW observatories.

Blazars are a subclass of active galactic nuclei (AGN) that emit non-thermal radiation through relativistic jets, characterized by rapid flux and polarization variability. High synchrotron-peaked blazars (HSPs) and extreme high synchrotron-peaked blazars (EHSPs), with synchrotron peaks exceeding $10^{15}$ Hz and $10^{17}$ Hz, respectively, are crucial for understanding the full range of blazar phenomena and testing models of jet physics. Yet, their understanding remains challenging. This work aims to systematically identify and characterize the most extreme $\gamma$-ray blazars using data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. The focus is on spectral hardening, where the $\gamma$-ray spectrum becomes harder at higher energies, particularly during flaring episodes. This represents the first dedicated analysis of spectral hardening across a population of EHSPs, as previous studies explored it only in individual sources. We analyze 138 blazars selected from the 4FGL-DR2 catalog with high synchrotron peak frequencies and well-sampled light curves. Flaring periods are automatically identified, and each flare is analyzed, with the significance of spectral hardening assessed through a test statistic based on the likelihood ratio of two spectral models. We identify two flaring episodes with indications of spectral hardening, in 4FGL J0238.4$-$3116 and PKS 2155$-$304, the latter detected independently by both methods but referring to the same period. These events are consistent with expectations from statistical fluctuations, suggesting that spectral hardening is a rare occurrence (< 0.1 %). These results constrain its frequency and support a smoothly varying power-law blazar emission model, motivating future multi-wavelength studies to clarify whether these rare flares reflect distinct physical processes within blazar jets.

In this work, we present LensingFlow. This is an implementation of an automated workflow to search for evidence of gravitational lensing in a large series of gravitational wave events. This workflow conducts searches for evidence in all generally considered lensing regimes. The implementation of this workflow is built atop the Asimov automation framework and CBCFlow metadata management software and the resulting product therefore encompasses both the automated running and status checking of jobs in the workflow as well as the automated production and storage of relevant metadata from these jobs to allow for later reproduction. This workflow encompasses a number of existing lensing pipelines and has been designed to accommodate any additional future pipelines to provide both a current and future basis on which to conduct large scale lensing analyses of gravitational wave signal catalogues. The workflow also implements a prioritisation management system for jobs submitted to the schedulers in common usage in computing clusters ensuring both the completion of the workflow across the entire catalogue of events as well as the priority completion of the most significant candidates. As a first proof-of-concept demonstration, we deploy LensingFlow on a mock data challenge comprising 10 signals in which signatures of each lensing regime are represented. LensingFlow successfully ran and identified the candidates from this data through its automated checks of results from consituent analyses.

We present an updated model for the extragalactic background light (EBL) from stars and dust, over wavelengths approximately 0.1 to 1000 $\mu$m. This model uses accurate theoretical stellar spectra, and tracks the evolution of star formation, stellar mass density, metallicity, and interstellar dust extinction and emission in the universe with redshift. Dust emission components are treated self-consistently, with stellar light absorbed by dust reradiated in the infrared as three blackbody components. We fit our model, with free parameters associated with star formation rate and dust extinction and emission, to a wide variety of data: luminosity density, stellar mass density, and dust extinction data from galaxy surveys; and $\gamma$-ray absorption optical depth data from $\gamma$-ray telescopes. Our results strongly constraint the star formation rate density and dust photon escape fraction of the universe out to redshift $z=10$, about 90% of the history of the universe. We find our model result is, in some cases, below lower limits on the $z=0$ EBL intensity, and below some low-$z$ $\gamma$-ray absorption measurements.

The global network of gravitational-wave detectors has completed three observing runs with $\sim 50$ detections of merging compact binaries. A third LIGO detector, with comparable astrophysical reach, is to be built in India (LIGO-Aundha) and expected to be operational during the latter part of this decade. Multiple detectors operating at different parts of the globe will provide several pairs of interferometers with longer baselines and an increased network SNR. This will improve the sky localisation of GW events. Multiple detectors simultaneously in operation will also increase the baseline duty factor, thereby, leading to an improvement in the detection rates and, hence, the completeness of surveys. In this paper, we quantify the improvements due to the expansion of the LIGO Global Network (LGN) in the precision with which source properties will be measured. We also present examples of how this expansion will give a boost to tests of fundamental physics.

The origin of the diffuse astrophysical neutrino flux observed by the IceCube experiment is still under debate. In recent years there have been associations of neutrino events with individual blazars, which are active galaxies with relativistic jets pointing toward Earth, such as the source TXS 0506+056. From a theoretical perspective, the properties of these sources as neutrino emitters are not yet well understood. In this work we model a sample of 324 blazars detected by the Fermi Large Area Telescope (LAT), most of which are flat-spectrum radio quasars (FSRQs). This amounts to 34% of all FSRQs in the latest Fermi catalog. By numerically modelling the interactions of cosmic-ray electrons and protons, we explain the emitted multi-wavelength fluxes from each source and self-consistently predict the emitted neutrino spectrum. We demonstrate that the optical and GeV gamma-ray broadband features are generally well described by electron emission. For 33% of the blazars in our sample, a description of the observed X-ray spectrum benefits from an additional component from proton interactions, in agreement with recent studies of individual IceCube candidate blazars. We conclude that blazars that are brighter in GeV gamma rays tend to have a higher neutrino production efficiency but a lower best-fit baryonic loading. The predicted neutrino luminosity shows a positive correlation with the observed GeV gamma-ray flux and with the predicted MeV gamma-ray flux. By extrapolating the results for this sample, we show that the diffuse neutrino flux from the population of gamma-ray-bright blazars may be at the level of about 20% of the IceCube flux, in agreement with current limits from stacking analyses. We discuss the implications of our results for future neutrino searches and suggest promising sources for potential detections with future experiments.

Lyman $\alpha$ (Ly$\alpha$) emission is one of few observable features of galaxies that can trace the neutral hydrogen content in the Universe during the Epoch of Reionization (EoR). To accomplish this we need an efficient way to survey for Ly$\alpha$ emitters (LAEs) at redshifts beyond 7, requiring unbiased emission-line observations that are both sufficiently deep and wide to cover enough volume to detect them. Here we present results from PASSAGE -- a pure-parallel JWST/NIRISS slitless spectroscopic survey to detect Ly$\alpha$ emitters deep into the EoR, without the bias of photometric preselection. We identify four LAEs at $7.5\leq z\leq9.5$ in four surveyed pointings, and estimate the luminosity function (LF). We find that the LF does show a marked decrease compared to post-reionization measurements, but the change is a factor of $\lesssim 10$, which is less than expected from theoretical calculations and simulations, as well as observational expectations from the pre-JWST literature. Modeling of the IGM and expected \lya\ profiles implies these galaxies reside in ionized bubbles of $\gtrapprox 2$ physical Mpc. We also report that in the four fields we detect {3,1,0,0} LAEs, which could indicate strong field-to-field variation in the LAE distribution, consistent with a patchy HI distribution at $z\sim8$. We compare the recovered LAE number counts with expectations from simulations and discuss the potential implications for reionization and its morphology.

The very high-energy (VHE; $>$100 GeV) radiation carries the signatures of the matter-energy interaction in some of the most extreme astrophysical environments. Considering broad emission line blazars, i.e., flat spectrum radio quasars (FSRQs), the dense photon fields surrounding the relativistic jet can prohibit the particle population from accelerating to very high energies and producing VHE radiation. They can also possibly make the environment opaque for the VHE $\gamma$ rays due to $\gamma\gamma$ pair production, thus explaining the paucity of VHE-detected FSRQs and non-detection of TeV radiation ($>$1 TeV) from them. Here we report, for the first time, a $>$7$\sigma$ detection of an FSRQ, S5 1027+74 ($z=0.123$), in the VHE band, including the first ever detection of TeV emission from an object of this class, using the Fermi Large Area Telescope observations. Its $\gamma$-ray spectrum covering the 100 MeV to 2 TeV band revealed a prominent spectral break with a flat, rising shape above $\sim$10 GeV, a feature never detected from other VHE-detected FSRQs. The radio-to-$\gamma$-ray spectral energy distribution of S5 1027+74 provides strong evidence of a third bump peaking at multi-TeV energies. These enigmatic findings imply that FSRQ jets can accelerate particles to extremely high energies and provide tantalizing clues about the complex radiative environment of relativistic jets.

Very-high energy (VHE; $>$100 GeV) $\gamma$-ray emission originates via some of the most extreme particle acceleration processes in the universe. Considering beamed active galactic nuclei, i.e., blazars, only a small fraction, mainly high synchrotron peak BL Lacs, have been detected in the VHE band with the ground-based Cherenkov telescopes. We utilized $\sim$16 years of Fermi-Large Area Telescope (LAT) observations in the 0.1$-$2 TeV energy range to systematically search for potential VHE emitters in a sample of high synchrotron peaked ($\nu^{\rm peak}_{\rm syn}>10^{15}$ Hz) BL Lac sources. We identified, for the first time, 92 VHE emitting blazars at $\geq 5\sigma$ confidence level. A significant VHE emission was also detected from 52 objects, which have been previously reported to be a VHE blazar. Comparing with the general blazar population, these VHE emitting blazars are found to be located at low redshifts (mean $z=0.2 \pm 0.1$) and exhibit bright synchrotron emission ($\log F^{\rm peak}_{\rm syn}=-11.2 \pm 0.4$, in erg cm$^{-2}$ s$^{-1}$). We also investigated the coincidence of VHE photon arrivals with the source activity states and found that Fermi-LAT has detected VHE photons during both quiescent and elevated activity epochs. These VHE emitting blazars represent promising targets for current and next-generation ground-based Cherenkov telescopes, and provide valuable laboratories for probing particle acceleration in relativistic jets, testing multi-messenger connections, and constraining extragalactic background light models.

Determining the Hubble constant (H0), a fundamental parameter describing cosmic expansion, remains a challenge due to conflicting measurements from the early and late universe. Gravitational wave (GW) observations from binary neutron star (BNS) mergers, with identified host galaxies through electromagnetic (EM) follow-up, offer an independent method to measure H0. However, this requires detection of numerous events, which could take decades with current GW detectors. LIGO-India can dramatically accelerate this effort. With sensitivity comparable to the existing LIGO detectors, its addition to the LIGO-Virgo network could increase detected events by 70%. This improvement nearly doubles when accounting for the detector's 70% duty cycle, increasing the probability of simultaneous operation of three detectors by a factor of ~2. We perform end-to-end simulations to estimate triple-coincidence detection rates and sky localization, considering realistic BNS populations, lightcurves, and EM observatory specifications. Our findings suggest LIGO-India could increase BNS events with observed kilonovae by ~2-7 times. The factor of few improvements in source localization precision with LIGO-India can allow much deeper EM follow-up campaigns (not considered in the simulations), potentially increasing the overall rate of detection of EM counterparts by a factor of ~20, which can have an enormous impact in addressing critical questions in different areas of astronomy. We evaluate the impact of LIGO-India in the context of H0 measurement and argue that it can cut down the required observation time of several decades by a factor of few and possibly to just few years with regular sensitivity upgrades.

We performed the spectral and temporal analysis of MAXI J1803-298 using AstroSat/LAXPC and NICER observations taken in May 2021 during the initial phase of the outburst. We found that the source traverses through the hard, intermediate, and soft spectral states during the outburst. The spectrum in all states can be described using soft emissions from the thermal disk and hard emissions from the coronal regions. The variation in the inner disk temperature and normalization of the disk indicates the motion of the truncated disk across these different spectral states. We confirmed the presence of broad features, Type-C, and Type-B QPOs in the power spectra of different spectral states. We investigated the fractional rms and lags of all the variability features and discovered that the lag swung between positive and negative during the outburst evolution. While modeling the features with a simple model that considers variations in accretion parameters such as the accretion rate, heating rate, and inner disk radius, along with delays between them, we found a dynamic reversal in the origin of variability between the corona and the disk. Furthermore, our results are consistent with previous works and a radio study conducted on this source during its outburst.

Next-generation GW detectors will produce a high rate of temporally overlapping signals from unrelated compact binary coalescences. Such overlaps can bias parameter estimation (PE) and mimic signatures of other physical effects, such as gravitational lensing. In this work, we investigate how overlapping signals can be degenerate with gravitational lensing by focusing on two scenarios: Type-II strong lensing and microlensing by an isolated point-mass lens. We simulate quasicircular binary black-hole pairs with chirp-mass ratios $\mathscr{M}_{\rm B}/\mathscr{M}_{\rm A}\in\{0.5,\,1,\,2\}$, SNR ratios $\mathrm{SNR}_{\rm B}/\mathrm{SNR}_{\rm A}\in\{0.5,\,1\}$, and coalescence-time offsets $\Delta t_{\rm c}\in[-0.1,\,0.1]~\mathrm{s}$. Bayesian PE and fitting-factor studies show that the Type-II lensing hypothesis is favored over the unlensed quasicircular hypothesis ($\log_{10}\mathscr{B}^{\rm L}_{\rm U}>1$) only in a small region of the overlapping parameter space with $\mathscr{M}_{\rm B}/\mathscr{M}_{\rm A}\gtrsim1$ and $|\Delta t_{\rm c}|\leq0.03~\rm{s}$.. Meanwhile, false evidence for microlensing signatures can arise because, to a reasonable approximation, the model produces two superimposed images whose time delay can closely match $|\Delta t_{\rm c}|$. Overall, the inferred Bayes factor depends on relative chirp-mass ratios, relative loudness, difference in coalescence times, and also the absolute SNRs of the overlapping signals. Cumulatively, our results indicate that overlapping black-hole binaries with nearly equal chirp masses and comparable loudness are likely to be falsely identified as lensed. Such misidentifications are expected to become more common as detector sensitivities improve. While our study focuses on ground-based detectors using appropriate detectability thresholds, the findings naturally extend to next-generation GW observatories.

Detection and classification of transients in data from gravitational wave detectors are crucial for efficient searches for true astrophysical events and identification of noise sources. We present a hybrid method for classification of short duration transients seen in gravitational wave data using both supervised and unsupervised machine learning techniques. To train the classifiers we use the relative wavelet energy and the corresponding entropy obtained by applying one-dimensional wavelet decomposition on the data. The prediction accuracy of the trained classifier on 9 simulated classes of gravitational wave transients and also LIGO's sixth science run hardware injections are reported. Targeted searches for a couple of known classes of non-astrophysical signals in the first observational run of Advanced LIGO data are also presented. The ability to accurately identify transient classes using minimal training samples makes the proposed method a useful tool for LIGO detector characterization as well as searches for short duration gravitational wave signals.

Low Mass X-ray binaries (LMXBs) are binary systems where one of the components is either a black hole or a neutron star and the other is a less massive star. It is challenging to unambiguously determine whether a LMXB hosts a black hole or a neutron star. In the last few decades, multiple observational works have tried, with different levels of success, to address this problem. In this paper, we explore the use of machine learning to tackle this observational challenge. We train a random forest classifier to identify the type of compact object using the energy spectrum in the energy range 5-25 keV obtained from the Rossi X-ray Timing Explorer archive. We report an average accuracy of 87+/-13 in classifying the spectra of LMXB sources. We further use the trained model for predicting the classes for LMXB systems with unknown or ambiguous classification. With the ever-increasing volume of astronomical data in the X-ray domain from present and upcoming missions (e.g., SWIFT, XMM-Newton, XARM, ATHENA, NICER), such methods can be extremely useful for faster and robust classification of X-ray sources and can also be deployed as part of the data reduction pipeline.

In this study, we examine the thermodynamics of black holes immersed in perfect fluid dark matter (PFDM) by employing the Misner-Sharp energy framework. We extend the analysis to include the thermal fluctuations of these black holes when size reduces to small size, redefining key thermodynamic variables such as pressure, volume, temperature, internal energy, and entropy within the context of PFDM. Using these newly defined variables, we systematically calculate the enthalpy, Gibbs free energy, and specific heat of PFDM black holes, incorporating perturbative thermal corrections. To assess the stability of these black holes, we perform a detailed graphical analysis of the specific heat as a function of the horizon radius, providing insights into the thermal stability of PFDM black holes. This comprehensive approach enhances our understanding of the thermodynamic properties and stability of black holes in the presence of dark matter.

Gravitational lensing offers a powerful probe into the properties of dark matter and is crucial to infer cosmological parameters. The Legacy Survey of Space and Time (LSST) is predicted to find O(10^5) gravitational lenses over the next decade, demanding automated classifiers. In this work, we introduce GraViT, a PyTorch pipeline for gravitational lens detection that leverages extensive pretraining of state-of-the-art Vision Transformer (ViT) models and MLP-Mixer. We assess the impact of transfer learning on classification performance by examining data quality (source and sample size), model architecture (selection and fine-tuning), training strategies (augmentation, normalization, and optimization), and ensemble predictions. This study reproduces the experiments in a previous systematic comparison of neural networks and provides insights into the detectability of strong gravitational lenses on that common test sample. We fine-tune ten architectures using datasets from HOLISMOKES VI and SuGOHI X, and benchmark them against convolutional baselines, discussing complexity and inference-time analysis.

We present a detailed comparative systematic study using a sample of 221 Narrow-line Seyfert 1 (NLSy1) galaxies in comparison to a redshift matched sample of 154 Broad-line Seyfert 1 (BLSy1) galaxies based on their observations using ROSAT and/or XMM-Newton telescopes in soft X-ray band (0.1-2.0 keV). A homogeneous analysis is carried out to estimate their soft X-ray photon indices ($\Gamma^{s}_{X}$) and its correlations with other parameters of nuclear activities such as Eddington ratios (R$_\mathrm{Edd}$), bolometric luminosities (L$_\mathrm{bol}$), black hole masses (M$_\mathrm{BH}$) and the widths of the broad component of H$\beta$ lines (FWHM(H$\beta$)). In our analysis, we found clear evidence of the difference in the $\Gamma^{s}_{X}$ and R$_\mathrm{Edd}$ distributions among NLSy1 and BLSy1 galaxies, with steeper $\Gamma^{s}_{X}$ and higher R$_\mathrm{Edd}$ for the former. Such a difference also exists in the spectral indices distribution in hard X-ray ($\Gamma^{h}_{X}$), based on the analysis of 53 NLSy1 and 46 BLSy1 galaxies in the 2-10 keV energy band. The difference in R$_\mathrm{Edd}$ distributions does exist even after applying the average correction for the difference in the inclination angle of NLSy1 and BLSy1 galaxies. We also estimated R$_\mathrm{Edd}$, based on SED fitting of 34 NLSy1 and 30 BLSy1 galaxies over the 0.3-10 keV energy band and found that results are still consistent with R$_\mathrm{Edd}$ estimates based on the optical bolometric luminosity. Our analysis suggests that the higher R$_\mathrm{Edd}$ in NLSy1 is responsible for its steeper X-ray spectral slope compared to the BLSy1, consistent with the disc-corona model as proposed for the luminous AGNs.

To an outside observer, a black hole's event horizon appears to behave exactly like a dynamical fluid membrane. We extend this membrane paradigm to black holes in general $f(R)$ theories of gravity. We derive the stress tensor and various transport coefficients of the fluid and find that the membrane behaves as a non-Newtonian fluid except for the special case of Einstein gravity. Using Euclidean methods, we study the thermodynamics of the membrane. We speculate on what theories of gravity admit horizons with fluid properties.

Giant radio galaxies (GRGs) are physically large radio sources that extend well beyond their host galaxy environment. Their polarization properties are affected by the poorly constrained magnetic field that permeates the intergalactic medium on Mpc scales. A low frequency (&lt; 200 MHz) polarization study of this class of radio sources is now possible with LOFAR. Here we investigate the polarization properties and Faraday rotation measure (RM) of a catalog of GRGs detected in the LoTSS. This is the first low-frequency polarization study of a large sample of radio galaxies selected on their physical size. We explore the magneto-ionic properties of their under-dense environment and probe intergalactic magnetic fields using the Faraday rotation properties of their radio lobes. We use RM synthesis in the 120-168 MHz band to search for polarized emission and to derive the RM and fractional polarization of each detected source component. We study the depolarization between 1.4 GHz and 144 MHz using images from the NVSS. From a sample of 240 GRGs, we detected 37 sources in polarization, all with a total flux density above 56 mJy. The fractional polarization of the detected GRGs at 1.4 GHz and 144 MHz is consistent with a small amount of Faraday depolarization (a Faraday dispersion &lt; 0.3 rad m$^{-2}$). Our analysis shows that the lobes are expanding into a low-density (&lt;10^{-5} cm$^{-3}$) local environment permeated by weak magnetic fields (&lt;0.1 $\mu$G) with fluctuations on scales of 3 to 25 kpc. The presence of foreground galaxy clusters appears to influence the polarization detection rate up to 2R$_{500}$. In general, this work demonstrates the ability of LOFAR to quantify the rarefied environments in which these GRGs exist and highlights them as an excellent statistical sample to use as high precision probes of magnetic fields in the intergalactic medium and the Milky Way.

We report the discovery of large ionized, [O II] emitting circumgalactic nebulae around the majority of thirty UV luminous quasars at $z=0.4-1.4$ observed with deep, wide-field integral field spectroscopy (IFS) with the Multi-Unit Spectroscopy Explorer (MUSE) by the Cosmic Ultraviolet Baryon Survey (CUBS) and MUSE Quasar Blind Emitters Survey (MUSEQuBES). Among the 30 quasars, seven (23%) exhibit [O II] emitting nebulae with major axis sizes greater than 100 kpc, twenty greater than 50 kpc (67%), and 27 (90%) greater than 20 kpc. Such large, optically emitting nebulae indicate that cool, dense, and metal-enriched circumgalactic gas is common in the halos of luminous quasars at intermediate redshift. Several of the largest nebulae exhibit morphologies that suggest interaction-related origins. We detect no correlation between the sizes and cosmological dimming corrected surface brightnesses of the nebulae and quasar redshift, luminosity, black hole mass, or radio-loudness, but find a tentative correlation between the nebulae and rest-frame [O II] equivalent width in the quasar spectra. This potential trend suggests a relationship between ISM content and gas reservoirs on CGM scales. The [O II]-emitting nebulae around the $z\approx1$ quasars are smaller and less common than Ly$\alpha$ nebulae around $z\approx3$ quasars. These smaller sizes can be explained if the outer regions of the Ly$\alpha$ halos arise from scattering in more neutral gas, by evolution in the cool CGM content of quasar host halos, by lower-than-expected metallicities on $\gtrsim50$ kpc scales around $z\approx1$ quasars, or by changes in quasar episodic lifetimes between $z=3$ and $1$.