arXiv subjects

Santabrata Das

Publications and source records attributed to Santabrata Das.

At least 19 recordsLinked to original sources

arXiv2026

Hybrid disc geometry for shocked accretion flows: Unveiling QPOs in black hole X-ray binaries

We investigate the efficacy of semi-analytical global accretion solutions in capturing the flow properties observed in two-dimensional numerical simulations of shocked accretion onto black holes. A comparative analysis reveals that no single disc geometry adequately matches the simulation profiles across the entire radial domain. The pre-shock region exhibits closer agreement with the conical disc geometry, while the post-shock region is better described by the vertical equilibrium disc, where enhanced thermal pressure leads to substantial vertical expansion. Motivated by these complementary behaviours, we introduce a hybrid disc geometry in which the pre-shock flow follows the conical solution and the post-shock flow attains vertical equilibrium. This hybrid model satisfactorily reproduces both dynamical and thermodynamical properties of shocked accretion flows with the predicted Mach number and temperature profiles closely matching the simulations and the inferred shock location differing by $\sim10\%$. Within this framework, we delineate the shock parameter space spanned by the energy ($\mathcal{E}$) and angular momentum ($λ$) of the flow for weakly and rapidly rotating black holes and investigate the possible origin of Quasi-periodic Oscillations (QPOs) in black hole X-ray binaries (BH-XRBs). We constrain flow parameters that reproduce observed QPO centroid frequencies ($ν_{\rm QPO}$) demonstrating that oscillations of the shock front provide a self-consistent mechanism for both low and high frequency QPOs. Extending the analysis to ten Galactic BH-XRBs, we demonstrate that the observed $ν_{\rm QPO}$ are reproduced within physically plausible parameter ranges, which establishes shocked global accretion solutions as a potentially compelling framework for interpreting accretion driven temporal variability.

astro-ph.HE↗

arXiv2026

Relativistic Accretion Flow in a Generic Class of Spherically Symmetric Static Spacetime

We investigate the properties of low angular momentum, inviscid, advective accretion flows in a generic static and spherically symmetric spacetime that incorporates higher-order corrections up to the fourth order in $1/r$. Employing this metric, we self-consistently solve the relativistic hydrodynamical equations and obtain the family of global transonic accretion solutions ($O$, $A$, $W$ and $I$-types) by means of the spacetime parameters ($δ$, $η$, $β$) and the flow parameters (specific energy $\mathscr{E}$ and angular momentum $λ$). Our analysis reveals that the accretion flow possesses either single or multiple critical points depending on these input parameters. We delineate the regions of the $δ-λ$ and $λ-\mathscr{E}$ parameter spaces that admits solutions with multiple critical points and demonstrate how these regions evolve with increasing spacetime parameter $δ$. Furthermore, while connecting the spacetime geometry with observable signatures, we compute the spectral energy distribution (SED) from thermal bremsstrahlung emission and observe that increasing $δ$ enhances the SED relative to the Schwarzschild case. Finally, we find that global transonic solutions harbouring inner critical points ($I$-types) yields more luminous power than those with only outer critical points ($O$ and $A$ types).

astro-ph.HE↗

arXiv2026

Influence of winds on shocked magnetized viscous accretion flows around rotating black holes

We study global transonic solution for a relativistic, magnetized, viscous advective accretion flow around a rotating black hole, incorporating the effects of mass and angular momentum loss through winds. Our model considers dominant toroidal magnetic fields with synchrotron radiation as the primary cooling mechanism. To self-consistently model mass loss, the mass accretion rate is prescribed to decrease inward as a power-law with disk radius. With this, we solve the governing equations that describe the accretion flows in presence of winds and obtain the flow structure in terms of the inflow parameters (energy $\mathcal{E}$, angular momentum $λ$, plasma-$β$, accretion rate $\dot{m}$, and viscosity $α_{\rm B}$), the wind parameters ($p$, governing mass loss; and $l$, governing angular momentum transport by winds), and the black hole spin ($a_{\rm k}$). Our analysis reveals that winds substantially modify the accretion flow leading to a significant decrease in disk luminosity. We specifically identify global solutions that admit standing shocks and find that winds profoundly alter shock properties, such as the shock radius ($x_{\rm s}$), compression ratio ($R$), and shock strength ($S$). Furthermore, we determine the critical wind parameter $p^{\rm crit}$ beyond which steady shock solutions cease to exist. We demonstrate that increased viscosity and strong angular momentum extraction by winds lead to reduce $p^{\rm crit}$. These findings evidently highlight a complex interplay between viscosity and winds in governing the dynamics of shock formation in accretion disks.

astro-ph.HE↗

arXiv2026

Transonic accretion flow in the mini discs of a binary black hole system

We study the general relativistic transonic accretion flow around the primary black hole, which forms the circumprimary disc (CPD), within a binary black hole (BBH) system. The BBH spacetime is characterized by the mass ratio ($q$) and the separation distance ($z_2$) between the two black holes. We numerically solve the radial momentum and energy equations to obtain the accretion solutions. It is observed that the CPD can exhibit shock solutions, which exist for a wide range parameter space spanned by flow specific angular momentum ($λ$) and energy ($E$). We find that the shock parameter space is modified by $q$ and $z_2$. Investigations show that $q$ and $z_2$ also affect various shock properties, such as density compression and temperature compression across the shock fronts. Moreover, we calculate the spectral energy distributions (SEDs) of the CPD and examine how the SEDs are modified by $q$ and $z_2$ for both shock-free and shock-induced accretion solutions. SED is found to be nearly independent of the binary parameters. We essentially show that although $q$ and $z_2$ alter the effective horizon area of the primary black hole located at the center of the CPD, they have a minimal impact on the dynamical and spectral properties of the accretion flow around the primary black hole.

astro-ph.HE↗

arXiv2025

Impact of general relativistic accretion on primordial black holes

We demonstrate that general relativistic corrections to the accretion of relativistic matter onto primordial black holes (PBHs) can significantly enhance their mass growth during the early Universe. Contrary to previous Newtonian treatments, our analysis reveals that PBH masses can increase by an order of magnitude before evaporation, leading to substantial modifications of their lifetime and cosmological imprints. We quantify the resulting shifts in the minimum PBH mass constrained by Big Bang Nucleosynthesis (BBN), the revised lower bound for PBHs surviving today, and the dark matter parameter space allowed by PBH evaporation. Furthermore, we show that the enhanced accretion alters the high-frequency gravitational wave spectrum from PBH evaporation, potentially within the reach of future detectors. Our results provide a comprehensive, relativistically consistent framework to delineate the role of PBHs in early-universe cosmology and dark matter phenomenology.

astro-ph.CO↗

arXiv2025

Neutrino-dominated relativistic shocked accretion flow around rotating black hole: implications for short gamma-ray bursts

We investigate the physical properties of the central engine powering gamma-ray bursts (GRBs), modelled as a stellar-mass black hole accreting via a neutrino-dominated accretion flow (NDAF). By solving the governing hydrodynamic equations, we obtain global transonic NDAF solutions featuring shock transitions and examine their role in powering GRB energetics. The NDAF solutions are explored over a broad range of black hole parameters, including its mass ($M_{\rm BH}$) and spin ($a_{\rm k}$), and accretion rate ($\dot{M}$). We find that shocked NDAFs can naturally account for the observed diversity in GRB energy output. Incorporating results from numerical simulations of binary neutron star and black hole-neutron star mergers, we estimate the remnant black hole mass and spin parameters for the predicted range of post-merger disk mass ($M_{\rm disk}$). Our analysis reveals that small-mass black holes with relatively low spin values can adequately reproduce the luminosities of short GRBs (SGRBs), whereas identical GRB luminosities can also be achieved for more massive black holes possessing higher spin values. Finally, we uncover a robust correlation between the black hole spin and disk mass such that $M_{\rm disk}$ decreases with increasing $a_{\rm k}$, remaining largely independent of the black hole mass ($M_{\rm BH}$) powering GRBs.

astro-ph.HE↗

arXiv2025

Dark matter halos and transonic accretion flow

The interplay between supermassive black holes (SMBHs) and their surrounding environment is fundamental to understanding galactic evolution. This work investigates the influence of a cold dark matter (DM) halo on the dynamics of relativistic, low angular momentum, inviscid, and advective hot accretion flow onto a galactic SMBH. Modeling the spacetime geometry as a black hole embedded within various DM distributions, including those with a central density spike, we demonstrate that the presence of a DM halo, particularly one that is massive and compact, enhances the luminosity of the accretion disk. The dominant contribution to this luminosity originates from the inner regions of the flow, suggesting that luminosity measurements could serve as a valuable observational probe for the dense DM environments expected near galactic centers.

gr-qc↗

arXiv2025

Probing the accretion geometry of black hole X-ray binaries: A multi-mission spectro-polarimetric and timing study

We present a comprehensive spectro-polarimetric and timing analysis of twelve black hole X-ray binaries, namely Cyg X-1, 4U 1630-47, Cyg X-3, LMC X-1, 4U 1957+115, LMC X-3, Swift J1727.8-1613, GX 339-4, Swift J151857.0-572147, IGR J17091-3624, MAXI J1744-294 and GRS 1915+105, using quasi-simultaneous observations from {\it{IXPE}}, {\it{NICER}}, {\it{NuSTAR}}, and {\it{AstroSat}}. Timing analyses reveal type-B and type-C Quasi-periodic Oscillations across different spectral states, often associated with episodic radio ejections. Broadband ($0.7-60$ keV) spectral modeling, employing disc, Comptonization, and reflection components, reveals degeneracies in constraining disc-corona geometries. Polarimetric measurements in $2-8$ keV band detect significant polarization degrees (PDs) ranging from $3-20.6\%$ ($1.2-21.4\%$) in harder (softer) states, with moderate to strong energy dependence, except for LMC X$-1$, Swift J$151857.0-572147$, and MAXI J1744$-$294, where no significant polarization is detected. We report polarization detections of Cyg X$-3$ (PD $\sim 21.4\%$, SIMS), LMC X$-3$ (PD $\sim 2.4\%$, HSS) and IGR J$17091-3624$ (PD $\sim 9\%$, LHS) using recent {\it IXPE} observations. A positive correlation is found between PD and Comptonized photon fraction ($cov_{\rm frac}$), while an anti-correlation is observed with disc-to-Comptonized flux ratio ($F_{\rm ratio}$) across spectral states. Combined timing, spectral, and polarimetric results, together with constraints from radio jet observations, suggest a radially extended corona within a truncated disc for Cyg X$-1$, Swift J$1727.8-1613$, IGR J$17091-3624$, and GX $339-4$, whereas the disc-corona geometry remains poorly constrained for 4U $1957+115$, LMC X$-3$, and 4U $1630-47$. We discuss the implications of these findings for understanding accretion geometries and highlight prospects for future X-ray polarimetric studies.

astro-ph.HE↗

arXiv2025

GRMHD modelling of accretion flow around Sagittarius A$^*$ constrained by EHT measurements

We study low angular momentum, advective accretion flows around a Kerr black hole within the framework of general relativistic magnetohydrodynamics (GRMHD) in the steady state. By solving the full set of GRMHD equations, we aim to provide a comprehensive understanding of the behavior of magnetized plasma in the strong gravity regime near a rotating black hole. The accretion solutions are obtained for a set of input parameters, namely energy (${\cal E}$), angular momentum (${\cal L}$), magnetic flux ($Φ$), and isorotation parameter ($I$). By systematically varying these parameters, we generate a family of global GRMHD accretion solutions that characterize the physical environment around the black hole. Using this approach, we investigate whether the inferred magnetic field strengths reported by the Event Horizon Telescope (EHT) for Sagittarius A$^*$ at various radii can be reproduced. We find that, for a broad range of parameter values, our model successfully recovers the EHT inferred magnetic field strengths with an accuracy of approximately $10\%$, offering a self-consistent framework for interpreting near-horizon accretion physics.

astro-ph.HE↗

arXiv2025

Possible detection of HFQPOs associated with 'unknown' variability class of GRS 1915+105

We present a comprehensive spectro-temporal analysis of GRS $1915+105$ observed with AstroSat during June, $2017$. A detailed study of the temporal properties reveals the appearance of an `unknown' variability class ($τ$) during $ρ\rightarrow κ$ class transition of the source. This new `unknown' class ($τ$) is characterized by the irregular repetition of low count `dips' along with the adjacent `flare' like features in between two successive steady count rate durations, resulting in uniform `$C$' shaped distribution in the color-color diagram. A detailed comparative study of the variability properties between the $τ$ class and other known variability classes of GRS $1915+105$ indicates it as a distinct variability class of the source. Further, we find evidence of the presence of possible HFQPO features at $\sim 71$ Hz with quality factor $\sim 13$, rms amplitude $\sim 4.69\%$, and significance $3σ$, respectively. In addition, a harmonic-like feature at $\sim 152$ Hz is also seen with quality factor $\sim 21$, rms amplitude $\sim 5.75\%$ and significance $\sim 4.7σ$. The energy-dependent power spectral study reveals that the fundamental HFQPO and its harmonic are present in $3-15$ keV and $3-6$ keV energy ranges, respectively. Moreover, the wide-band ($0.7-50$ keV) spectral modelling comprising of thermal Comptonization component indicates the presence of a cool ($kT_{\rm e}\sim 1.7$ keV) and optically thick (optical depth $\sim 14$) Comptonizing `corona', which seems to be responsible in regulating the HFQPO features in GRS $1915$+$105$. Finally, we find the bolometric luminosity ($L_{\rm bol}$) to be about $42\% L_{\rm Edd}$ within $1-100$ keV, indicating the sub-Eddington accretion regime of the source.

astro-ph.HE↗

arXiv2025

Study of mass outflows from magnetized accretion disks around rotating black holes with thermal conduction

We examine mass outflows from a low-angular momentum, viscous, advective, and magnetized accretion disk around a rotating black hole in presence of thermal conduction. We consider the disk is primarily threaded by the toroidal component of the magnetic field and an effective potential satisfactorily mimicked the spacetime geometry around the rotating black hole. With this, we self-consistently solve the coupled governing equations for inflow and outflow and compute the mass outflow rate $R_{\dot m}$ (ratio of mass flux of inflow to outflow) in terms of the inflow parameters, namely energy ($\mathcal{E}$), angular momentum ($λ$), plasma-$β$ and conduction parameter ($Υ_{\rm s}$) around weakly rotating ($a_{\rm k} \rightarrow 0$) as well as rapidly rotating ($a_{\rm k} =0.99$) black holes. Our findings reveal that the present formalism admits coupled inflow-outflow solutions across a wide range of inflow parameters yielding substantial mass loss. We observe that $R_{\dot m}$ monotonically increases with $Υ_{\rm s}$, irrespective of black hole spin. We also find that for a fixed $Υ_{\rm s}$, when energy, angular momentum, and magnetic field strength of the inflowing matter is increased, $R_{\dot{\rm m}}$ is enhanced resulting the outflows even more pronounced. We further estimate the maximum outflow rate ($R^{\rm max}_{\dot{\rm m}}$) by varying the inflow parameters and find that thermal conduction leads to maximum mass outflow rate $R^{\rm max}_{\dot{\rm m}} \sim 25\%$ for rapidly rotating black hole of spin $a_{\rm k} = 0.99$. Finally, we employ our formalism to explain the kinetic jet power of $68$ radio-loud low-luminosity active galactic nuclei (LLAGNs), indicating that it is potentially promising to account for the observed jet power of substantial number of LLAGNs.

astro-ph.HE↗

arXiv2025

Effect of generic dark matter halo on transonic accretion onto galactic black holes

The environment surrounding a black hole or black hole binaries is generally expected to play an important role in understanding various astrophysical phenomena around them. In this paper, we study relativistic, low angular momentum, inviscid, and advective hot accretion flow onto a galactic supermassive black hole dressed with a cold dark matter halo. Focusing on different relativistic dark matter distributions with an inner density spike, we analyze the effect of the dark matter halo on the topology and properties of the accretion flow. Our results show enhancement of disk luminosity in the presence of dark matter, which depends on the nature and properties (halo mass and compactness) of the dark matter distribution. Under the assumptions of our accretion model, the dominant contribution to the disk luminosity for compact and massive halos arises from the inner regions of the accretion flow. Consequently, our analysis indicates that luminosity measurements can serve as an effective probe of the underlying dark matter density spike.

gr-qc↗

arXiv2025

Revisiting Disc-Jet Coupling in Black Hole X-ray Binaries: On the Nature of Disc Dynamics and Jet Velocity

We perform a comprehensive wide-band ($3-100$keV) spectro-temporal analysis of 13 outbursting BH-XRBs, using data (quasi)simultaneous with radio observations to unravel the complex disc-jet connection. RXTE observations are analyzed for XTEJ1859+226, GX339-4 (2002, 2006, and 2010 outbursts), 4U1543-47, H1743-322 (2003 and 2009 outbursts), XTEJ1550-564, XTEJ1752-223, XTEJ1650-500, SwiftJ1753.5-0127, XTEJ1748-288, and GROJ1655-40. For SwiftJ1727.8-1613 and MAXIJ1535-571, we utilize HXMT data, while both AstroSat and HXMT observations are analyzed for SwiftJ1658.2-4242. Type-C QPOs observed in harder states (LHS, HIMS; $F_{nth}\ge0.4$) exhibit positive lag for low-inclination sources ($i<50^{\circ}$), whereas it generally exhibits negative lag for high-inclination sources ($i>60^{\circ}$), except XTEJ1550-564, SwiftJ1727.8-1613, H1743-322 (2003 outburst) and GROJ1655-40. Notably, type-A QPOs exhibit negative lags ($\sim1-10$ms) regardless of source inclination, while type-B QPOs show positive lags in low-inclination sources, and both positive and negative lags ($\sim1-15$ms) in high-inclination sources, typically occurring in SIMS ($F_{nth}\lesssim0.45$). Systematic appearance of type-A QPOs preceding radio flares in several sources suggests that type-A QPOs indicate telltale signs of jet ejection, while type-B QPOs are closely linked with radio flares (i.e., transient jets). Present findings suggest the corona evolves from a radially extended to a vertically elongated structure during the type-C to type-B transition via type-A QPOs, with type-B QPOs linked to radially compact or vertically extended coronal geometries, resembling jet ejection. The strong radio-X-ray luminosity correlation seems to provide compelling evidence of accretion-powered jets. Finally, we find that jets in SIMS are moderately relativistic in nature with velocities $\gtrsim 0.3-0.8c$ in BH-XRBs under consideration.

astro-ph.HE↗

arXiv2025

Evidence of oscillating `compact' Comptonized corona in GRS 1915+105: Insights into HFQPOs with AstroSat

We present, for the first time, an in-depth dynamical analysis of the spectro-temporal properties of the soft variability classes ($δ$, $κ$, $ω$, and $γ$) of GRS 1915+105 during the detection of $\sim$70 Hz High-Frequency Quasi-periodic Oscillations (HFQPOs) using AstroSat data. The wide-band spectra ($0.7-50$ keV) are well described by thermal Comptonization along with an extended power-law component. Additionally, power spectra ($0.01-500$ Hz) indicate that Comptonized photons ($6-25$ keV) primarily contribute to the HFQPOs. Our findings reveal that high (low) count rates referred to as `non-dips' (`dips') in the light curves of the variability classes correspond to the detection (non-detection) of HFQPOs. Accumulated `non-dips' (`dips') spectra are modelled separately using thermal Comptonization (\texttt{nthComp}) as well as \texttt{kerrd} which indicates harder spectra and smaller inner disc radius during the detection of HFQPOs. We conduct dynamical analyses (every 32 s) to trace the presence of HFQPOs, and variations in thermal Comptonization parameters ($Γ_{\rm nth}$ and ${\rm N}_{\rm nth}$). Moreover, we observe a positive correlation of `non-dips' with QPO strength, ${\rm HR}1$, and ${\rm N}_{\rm nth}$, while $Γ_{\rm nth}$ shows an anti-correlation, suggesting that high-energy photons from the Comptonized corona are responsible for the HFQPOs. Furthermore, we estimate the size of the Comptonized corona using \texttt{kerrd} and \texttt{diskpn} to be $\sim 2.8 - 16$ $r_{\rm g}$. Thus, we infer that a `compact' oscillating corona likely modulates the high-energy radiation, exhibiting the $70$ Hz HFQPOs in GRS 1915$+$105.

astro-ph.HE↗

arXiv2025

Astrophysics with Compact Objects: An Indian Perspective, Present Status and Future Vision

Astrophysical compact objects, viz., white dwarfs, neutron stars, and black holes, are the remnants of stellar deaths at the end of their life cycles. They are ideal testbeds for various fundamental physical processes under extreme conditions that are unique in nature. Observational radio astronomy with uGMRT and OORT facilities has led to several important breakthroughs in studies of different kinds of pulsars and their emission mechanisms. On the other hand, accretion processes around compact objects are at the core of Indian astronomy research. In this context, AstroSat mission revolutionized spectro-temporal observations and measurements of accretion phenomena, quasi-periodic oscillations, and jet behaviour in binary systems hosting compact objects. Moreover, recently launched XPoSat mission is set to provide an impetus to these high-energy phenomena around compact objects by enabling us to conduct polarization measurements in the X-ray band. Further, during the past decade, numerous gravitational wave signals have been observed from coalescing black holes and neutron stars in binary systems. Recent simultaneous observation of the GW170817 event in both gravitational waves and electromagnetic channels has ushered in the era of multi-messenger astronomy. In the future, synergistic efforts among several world-class observational facilities, e.g., LIGO-India, SKA, TMT, etc., within the Indian astrophysics community will provide a significant boost to achieve several key science goals that have been delineated here. In general, this article plans to highlight scientific projects being pursued across Indian institutions in this field, the scientific challenges that this community would be focusing on, and the opportunities in the coming decade. Finally, we have also mentioned the required resources, both in the form of infrastructural and human resources.

astro-ph.HE↗

arXiv2025

Effect of thermal conduction on accretion shocks in relativistic magnetized flows around rotating black holes

We examine the effects of thermal conduction on relativistic, magnetized, viscous, advective accretion flows around rotating black holes considering bremsstrahlung and synchrotron cooling processes. Assuming the toroidal component of magnetic fields as the dominant one, we self-consistently solve the steady-state fluid equations to derive the global transonic accretion solutions for a black hole of spin $a_{\rm k}$. Depending on the model parameters, the magnetized accretion flow undergoes shock transitions and shock-induced global accretion solutions persist over a wide range of model parameters including the conduction parameter ($Υ_{\rm s}$), plasma-$β$, and viscosity parameter ($α_{\rm B}$). We find that the shock properties -- such as shock radius ($r_{\rm s}$), compression ratio ($R$), and shock strength ($S$) -- are regulated by $Υ_{\rm s}$, plasma $β$, and $α_{\rm B}$. Furthermore, we compute the critical conduction parameter ($Υ_{\rm s}^{\rm cri}$), a threshold beyond which shock formation ceases to exist, and investigate its dependence on plasma-$β$ and $α_{\rm B}$ for both weakly rotating ($a_{\rm k} \rightarrow 0$) and rapidly rotating ($a_{\rm k} \rightarrow 1$) black holes. Finally, we examine the spectral energy distribution (SED) of the accretion disc and observe that increased thermal conduction and magnetic field strength lead to more luminous emission spectra from black hole sources.

astro-ph.HE↗

arXiv2025

Long-term evolution of BH-ULX candidates: An `unusual' $L_{\rm disc}$-$T_{\rm col}$ correlation associated with spectral states

We present the long-term spectral evolution of eight black hole ultra-luminous X-ray sources (BH-ULXs), namely NGC1313 X-1, NGC5408 X-1, NGC6946 X-1, IC342 X-1, NGC55 ULX1, NGC4395 ULX1, NGC5204 X-1 and NGC4190 ULX1 using {\it XMM-Newton} monitoring data spanning over a decade or more. An in-depth spectral modeling with thermal Comptonization ({\it nthComp}) and standard disc ({\it diskbb}) components reveals NGC5204 X-1, IC342 X-1, NGC4190 ULX1 and NGC1313 X-1 exhibiting harder spectral characteristics with dominant effect of Comptonization ($F_{nth}>F_{disc}$, $Γ_{nth}\lesssim2$). However, NGC6946 X-1 and NGC55 ULX1 remain in a disc-dominated state ($F_{disc}\sim2F_{nth}$, $Γ_{nth}\gtrsim2$), while NGC5408 X-1 shows intermediate spectral characteristics. The spectral analyses indicate an anti-correlation between disc luminosity ($L_{disc}$) and temperature ($T_{col}$) for all sources except NGC5204 X-1. These anti-correlations follow a relation $L_{disc}\propto T_{col}^α$ with steeper exponents of $α=-6.01\pm0.25$ (NGC55 ULX1), $-8.93\pm0.11$ (NGC6946 X-1), and $-10.31\pm0.10$ (NGC5408 X-1) for sources with softer or intermediate spectral characteristics. For harder sources, NGC1313 X-1 and IC342 X-1, the combined results provide $α=-3.58\pm0.04$. However, for NGC5204 X-1, a positive correlation is found, yielding $α=1.4\pm0.1$, suggesting that the emission mechanism is associated with the transition from the `standard disc' to the `slim disc' scenario. These findings suggest that the observed $L_{disc}-T_{col}$ correlations, along with the overall spectro-temporal properties of BH-ULXs, seems to be governed by disc-corona-wind driven accretion processes at various inclinations. Finally, we report a QPO-like feature ($\sim20$ mHz) with $rms\%\sim6.6$, Q-factor $\sim6.7$ and significant $2.8σ$ in NGC55 ULX1.

astro-ph.HE↗

arXiv2025

Neutrino-dominated relativistic viscous accretion flows around rotating black holes with shocks

We investigate the relativistic, viscous, advective, neutrino-dominated accretion flows (NDAFs) around rotating stellar mass black holes, incorporating neutrino cooling. By adopting an effective potential to describe the spacetime geometry around the rotating black holes, we self-consistently solve the governing NDAF equations to obtain global transonic accretion solutions. Our findings indicate that, depending on the model parameters, namely energy ($\varepsilon$), angular momentum ($λ$), accretion rate ($\dot{m}$), viscosity ($α$) and black hole spin ($a_{\rm k}$), NDAFs may harbor standing shocks where the Rankine-Hugoniot shock conditions (RHCs) are satisfied. Utilizing these shock-induced NDAF solutions, we compute the neutrino luminosity ($L_ν$) and neutrino annihilation luminosity ($L_{ν\barν}$) across a wide range of model parameters. We further calculate maximum neutrino luminosity ($L_ν^{\rm max}$) and neutrino annihilation luminosity ($L_{ν\barν}^{\rm max}$) resulting in $L_ν^{\rm max} \sim 10^{51-53}$ erg s$^{-1}$ ($10^{48-51}$ erg s$^{-1}$) and $L_{ν\barν}^{\rm max} \sim 10^{48-52}$ erg s$^{-1}$ ($10^{42-49}$ erg s$^{-1}$) for $a_{\rm k}=0.99$ (0.0). These findings suggest that shocked NDAF solutions are potentially promising to explain the energy output of gamma-ray bursts (GRBs). We employ our NDAF model formalism to elucidate $L^{\rm obs}_{ν\barν}$ for five GRBs with known redshifts and estimate their accretion rate (${\dot m}$) based on the spin ($a_{\rm k}$) of the central source of GRBs under consideration.

astro-ph.HE↗