We present the results of a comprehensive long-term spectro-temporal analysis of eight ultra-luminous X-ray sources (ULXs) with the central object being a black hole, using XMM-Newton monitoring of about a decade. Temporal studies reveal the existence of short-term variability in each sources with fractional variance varying in the range of $1.42-27.28$ per cent. Five sources of the sample are...
X-ray binaries, the most luminous galactic objects, harbor the universe's fascinating Neutron Stars (NSs) and Black Holes (BHs). In particular, low-mass X-ray binaries (LMXBs), in which matter from a solar-like companion star falls towards the compact object via an accretion disc, represent excellent laboratories to investigate the motion of matter orbiting nearby these extreme objects. In...
Tidal disruption of white dwarfs (WD) is an astrophysical transient phenomenon capable of revealing significant information on the presence of intermediate-mass black holes (IMBH). Using hydrodynamical simulations, we explore the observable properties of these astrophysical events. We accurately calculate the fallback rate of the tidal debris during its accretion onto an IMBH. We find the...
The recent observations of the shadows and images of the supermassive compact objects Sgr A and M87 at the hearts of Our Galaxy and the nearby galaxy M87, respectively, by The Event Horizon Telescope (EHT) collaboration have opened up a new window in observational astronomy to probe and test gravity and fundamental physics in the strong-field regime. It is commonly believed that the...
The quest to understand the properties of matter at high density has intrigued physicists for more than a few decades. The problem is complicated, as having a proper theory describing it is challenging. Also, earth-based experiments to probe them have yet to materialize. One of the naturally occurring laboratories where such matter exists is the cores of a neutron star. Recent precise mass and...
The study of the interior regions of a neutron star is one of the active areas of research
and gravitational wave astronomy is one of its critical tools. Currently, astronomers
from across the spectrum are detecting different neutron star systems and it has
become essential to consistently combine this information. We perform Bayesian
inference to constrain the equation of state of the...
A class of relativistic compact objects is analysed in modified Finch-Skea geometry described by modified MIT bag model equation of state of the interior matter. The bag constant B plays an important role in determining the physical features of strange star. In this work we have considered the effect of finite strange quark ($m_{s}≠ 0$) on the stability of quark matter inside a star. We have...
This abstract is based on our recent papers PRD 105 (2022) 024028 and PRD 107 (2023) 044072. Recent observations of several peculiar over- and under-luminous type Ia supernovae infer indirect evidences for the violation of the Chandrasekhar mass-limit by suggesting the existence of super- and sub-Chandrasekhar limiting mass white dwarfs (WDs). In an attempt to explain these phenomena in the...
To understand the decaying phase of outbursts in black hole (BH) X-ray binaries (BH-XRBs), we performed very long general relativistic magneto-hydrodynamic (GRMHD) simulations of a geometrically thin accretion disk around a Kerr BH with slowly rotating matter injected from outside. We thoroughly studied the flow properties, dynamical behavior of the accretion rate, magnetic flux rate, and jet...
We study a global, two-dimensional (2D) general relativistic magnetohydrodynamics (GRMHD) simulation of an accreting torus around a non-rotating black hole. Our initial configuration is threaded with a net-vertical magnetic flux. This study investigates the effects of initial field strength onto the disk dynamics. We find that the initial net vertical magnetic field significantly enhances its...
We investigate accretion flows around rotating black holes (BHs) and obtain self-consistent transonic solutions in full general relativistic prescription. The flow is assumed to be viscous and radiative. Viscosity helps in the removal of angular momentum outwards, allowing matter to get accreted inwards. In addition, viscous heat dissipated makes the matter hotter. On the other hand, radiation...
Most galaxies host a supermassive black hole (SMBH) at their centers, yet a large fraction of SMBHs are quiescent and only observable through their transient interactions with stellar objects. As a star approaches an SMBH, the tidal gravitational forces can overwhelm the star's self-gravity, resulting in a tidal disruption event (TDE). The disrupted debris accretes onto the black hole,...
Space observatory Gaia has prepared a large catalog of 1 billion astronomical objects which consists primarily of stars but also planets, comets, asteroids and quasars among others whose parallaxes are measured instead of distances ($r$). Bailer-Jones (\citet{Bailer-Jones}) established that distance estimation from parallaxes is not trivial once the fractional parallax error is larger...
When the fuel of a very massive star is spent, it collapses due to its own gravitational pull and eventually becomes a very small region of arbitrarily high matter density, that is a 'Singularity', where the usual laws of physics may break down. If this singularity is hidden within an event horizon, which is an invisible closed surface from which nothing, not even light, can escape, then we...
We explore the behaviour of barotropic and irrotational fluids with a small viscosity under the effect of first-order acoustic perturbations. We discuss, following the extant literature, the difficulties in gleaning an acoustic geometry in the presence of viscosity. In order to obviate various technical encumbrances, when viscosity is present, for an extraction of a possible acoustic geometry,...
GRS 1716-249 is a transient galactic black hole that experienced nine months of outburst activity in 2016–17. During its outbursts, Astrosat observed it at three different epochs with a fair amount of exposure time. We investigate whether quasi-periodic oscillation (QPO) exists and how it has evolved throughout the three epochs. We also explore the energy-dependent nature of QPOs. We model the...
We present the comprehensive analyses of the wide band spectral and timing variabilities of the two BH sources, namely GX 339$-$4 and H 1743$-$322, using AstroSat and NuSTAR observations during $2016-2022$. We observe that both GX 339$-$4 and H 1743$-$322 experienced successful as well as failed outbursts during $2016-2022$. Using long-term MAXI and BAT light curves, we examine the outburst...
Over the last few years, there has been considerable interest in massive compact stars, specifically neutron stars (NSs) and white dwarfs (WDs). Peculiar over-luminous type Ia supernovae (such as SNLS-03D3bb) and gravitational wave observations (such as GW190814) lend observational support to this idea. These massive compact stars are also prime candidates to fill in the observational mass gap...
A detailed analysis is presented of the gravitational microlensing by intervening compact objects of the black hole shadows imaged by the Event Horizon Telescope (EHT). We show how the center, size, and shape of the shadow depend on the Einstein angle relative to the true/unlensed shadow size, and how the location of the lens affects the shift, size, and asymmetry of the black hole shadow due...
The LIGO/Virgo detections showed unexpected progenitor black hole masses (~66 solar mass). Such black holes with their mass falling in the pair instability mass-gap region seek a new formation channel. We focus on the so-called AGN channel to understand such a puzzling progenitor mass. In this study, we numerically model 3D global MHD accretion flows of embedded black holes within a turbulent...
We demonstrate a general relativistic approach to model dark matter halos using the Einstein cluster, with the matter stress-energy comprising collisionless particles moving on circular geodesics in all possible
angular directions and orbital radii. Such matter, as is known, allows an anisotropic pressure profile with non-zero tangential but zero radial pressure. We use the Einasto density...
We study the equilibrium configurations and the radial stability of spherically symmetric relativistic Neutron Stars(NS) with a polytropic equation of state (EoS) in a modified $f (R, T )$ gravity framework by introducing a quadratic term in $T$ (where $T$ is the trace of the conserved energy-momentum tensor $T_{\mu \nu}$ of the matter-energy) for the functional form of $f(R, T)$ with $f (R, T...
Abstract: We study the properties of the neutrino-dominated accretion flow (NDAF) around the rotating black holes. The accretion flow of this kind involves hyper-accretion rate (${\dot m} \sim 0.001-10$ $M_\odot~{\rm s}^{-1}$) and because of this, the disk becomes geometrically as well as optically thick that makes it difficult for photons to escape. On the contrary, neutrinos easily move out...
We present the axisymmetric numerical simulations of the relativistic transonic jets around black holes driven by the radiation field of the accretion disk. We show that starting from a very low velocity at the base, jets can be accelerated to relativistic terminal speeds. Our results show the morphology of the jets during their different evolutionary stages. In addition to acceleration, the...
We investigate the time evolution of the transonic-viscous accretion flow around a non-rotating black hole. The input parameters used for the simulation are obtained from semi-analytical solutions. This code is based on the TVD routine and correctly handles the angular momentum transport due to viscosity. The thermodynamic properties of the flow are described by an equation of state with a...
It is widely postulated that the high-energy radiation in X-ray binary systems arises from the upscattering of photons originating from the accretion disk, likely due to interactions with an electron cloud or a corona. Nevertheless, our understanding of the exact geometry and orientation of these coronal structures remains limited. In recent times, spectro-polarimetry studies have emerged as a...
We study the relativistic, inviscid, advective accretion flow in a stationary axisymmetric Kerr-like wormhole spacetime characterized by the spin parameter ($a_{\rm k}$) and the dimensionless parameter ($\beta$). While doing this, we solve the governing equations that describe the relativistic accretion flow in a Kerr-like wormhole and calculate all types of accretion solutions, including...
We revisit the black hole X-ray binary source XTE J1859+226 during its outburst phase in 1999-2000 and carry out the spectral and timing analyses using RXTE observations. Over the course of outburst, type-B QPO is observed multiple occasions and the combined spectro-temporal results reveal enhanced hard X-ray contributions as ${\rm QPO}_{\rm rms}\% \sim 1-3$, covering fraction $\sim 0.4-0.6$...
The study aimed to examine the connection between radio pulsars and ultra-high energy neutrinos using the IceCube catalog of point source neutrino events. For this purpose we use the unbinned maximum likelihood method to search for a statistically significant excess from each of the pulsars in the ATNF catalog.
Next, we performed a Stacked search to further *enhance signal to noise...
The existence of black holes is one of the most astonishing predictions of general theory of relativity. The Event Horizon Telescope (EHT) collaboration's latest observations, as well as the discovery of gravitational wave signals by the Laser-Interferometer Gravitational Wave-Observatory (LIGO) and Virgo corroborate the existence of these celestial objects. Despite the success, there are...
We obtain the global properties of static and slowly rotating self-gravitating Bose-Einstein condensate (BEC) stars and study the effect of temperature on the stellar structural properties. For this we consider a recently developed temperature dependent equation of state of BEC stars formed due to Cooper pairing of nucleons. We use the Hartle-Thorne slow rotation approximation equations to...
We report the analysis of the Z-track neutron star (NS) low-mass X-ray binary (LMXB) GX 17+2 using the simultaneous data from the AstroSat (LAXPC/SXT) and NICER mission data. On segmenting the hardness intensity diagram (HID) into three slices—horizontal branch (HB), hard apex (HA), and normal branch (NB)- we investigate the variability of the source and its spectral state evolution throughout...
The study of accretion flow for astrophysical sources like active galactic nuclei (AGNs), black hole X-ray binaries (BHXRBs), etc., is essential to understand their spectral features. Nowadays, theorists hugely focused on the alternative gravity theory to explain some distinctive observational results from the usual Kerr BH. One such emerging non-Kerr spacetime is the Johannsen-Psaltis (JP)...
Superradiance in spinning black holes is an intriguing phenomenon through which the black hole loses its energy and angular momentum over time. In this work, we explore the transient effect of the superradiance process in active galactic nuclei (AGN). We aim to see the spin-down effect on the accretion disk using an analytic model named Shakura Sunayev. Considering this model, we show how...
Observation of Blackhole shadows by the VLBI Telescopes provides an excellent opportunity to test General Relativity in the strong field regime. One can use these observations to test the fundamental foundations of GR, such as Einstein's equivalence principle. In simple terms, Einstein's equivalence principle refers to simply changing the partial to covariant derivatives of the matter fields....
We devise a full general relativistic formalism to study the delays caused by the light bending effect in the signal of a radio pulsar in a binary. This delay is non negligible for neutron star - neutron star binaries and even stronger for neutron star - black hole binaries. We calculate bending delays for hypothetical neutron star - black hole binaries. The values of the bending delays...
Compact stars are unique laboratory for studying and testing extreme conditions in terms of density and gravity. In this paper under the framework of General Relativity we have developed a theoretical model representing compact stellar objects. For this we follow the Herrera's vanishing complexity condition in addition of assuming a particular geometry corresponding to g_rr component. All...
Ultra-high energy cosmic rays (UHECRs) beyond the Greisen-Zatsepin-Kuzmin (GZK) cut-off provide us with a unique opportunity to understand the universe at extreme energies. Secondary GZK photons and GZK neutrinos associated with the same interaction are indeed interconnected and render access to multi-messenger analysis of UHECRs. The GZK photon flux is heavily attenuated due to the...
This abstract is primarily based on ApJ 949 (2023) 62. Understanding various physical mechanisms requires an understanding of fundamental constants, however, measurements of these constants are subject to error due to experimental constraints. Researchers have proposed several bounds on fundamental constants based on a variety of experiments and observations. These constraints are different...
