arXiv subjects

Santabrata Das

Publications and source records attributed to Santabrata Das.

At least 37 records · Page 2Linked to original sources

arXiv2024

Study of a Tilted Thin Accretion Disk around a Kerr-Taub-NUT black hole

The accreting collapsed object GRO J1655-40 could contain the gravitomagnetic monopole (GMM), and it was shown to be better described by the Kerr-Taub-NUT (KTN) spacetime instead of the Kerr spacetime. The warped accretion disk has also been observed for the same collapsed object. Motivated by these, we study a tilted thin inner accretion disk around a KTN black hole. Such a tilting could have a significant effect on the X-ray spectral and timing features via the Lense-Thirring effect. Taking into account the contribution from the inner accretion disk for the KTN black hole, here we calculate the radial profile of a tilt angle. Depending on the numerical values of the viscosity of the accreting material and Kerr parameter, GMM tends the angular momentum of the disk to align along the black hole's spin axis, or to make it more tilted. Our solution for the radial profile of the tilted disk around a KTN black hole could be useful to probe the strong gravity regime, and could also give indirect evidence for the existence of GMM in nature.

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arXiv2024

General relativistic viscous accretion flow around Konoplya-Zhidenko black hole

We investigate the properties of accretion flows around the Konoplya-Zhidenko (KZ) black hole, which is proposed by deforming the Kerr metric with a single deformation parameter to test the no-hair theorem using gravitational wave observations. The dynamical equations describing the general relativistic viscous accreting flow are solved self-consistently to find the transonic accretion solutions in terms of global constants, such as energy ($E$), angular momentum ($\mathcal{L}$), viscosity parameter ($α$), spin ($a_{k}$), and deformation parameter ($η_0$). We obtain five distinct types of accretion solutions (O, A, $\text{A}^{\prime}$, W, and I-types), and observe that those solutions are not unique but rather continue to exist for wide range of parameter spaces in the $\mathcal{L}-E$ plane. Furthermore, we find that the viscous accretion flows can harbor shock waves when the relativistic shock conditions are satisfied. Consequently, the shock-induced global accretion solutions are obtained, and the effect of $η_0$ on shock properties, such as shock radius ($r_{\rm sh}$) and change in electron temperature ($T_{\rm e}$) across the shock front are investigated. Moreover, we calculate the spectral energy distributions (SEDs) of accretion flow using the relativistic thermal bremsstrahlung emission coefficient and study the modification of SEDs due to the increase of $η_0$ for both shock-induced and shock-free solutions. In addition, it has been noticed that the observable quantities, like quasi-periodic oscillation frequency ($ν_{\rm QPO}$) and bolometric disc luminosity ($L$), are strongly dependent on $η_0$. Finally, we phenomenologically show that the KZ black hole is consistent with the high-frequency QPOs, commonly observed in black hole binaries and black hole candidates.

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arXiv2024

Role of thermal conduction in relativistic hot accretion flow around rotating black hole with shock

We investigate the properties of low angular momentum, relativistic, viscous, advective accretion flows around rotating black holes that include shock waves in the presence of thermal conduction. We self-consistently solve the governing fluid equations to obtain the global transonic accretion solutions for a set of model parameters, namely energy ($\mathcal{E}$), angular momentum ($λ$), viscosity ($α$), conduction parameter ($Φ_{\rm s}$) and cooling parameter ($f_{\rm c}$). We observe that depending on the model parameters, accretion flow experiences centrifugally supported shock transition and the present study, for the first time, focuses on examining the shock properties, such as shock radius ($r_{\rm s}$), compression ratio ($R$) and shock strength ($Ψ$) regulated by the dissipation parameters ($Φ_{\rm s},~f_{\rm c}$). We show that shock-induced global accretion solutions persist for wide range of model parameters and identify the boundary of the parameter space in energy-angular momentum plane that admits standing shocks for different dissipation parameters ($Φ_{\rm s},~f_{\rm c}$). Finally, we compute the critical conduction parameter ($Φ_{\rm s}^{\rm cri}$), beyond which shock ceases to exist. We find that $Φ_{\rm s}^{\rm cri}$ directly depends on the black hole spin ($a_{\rm k}$) with $Φ_{\rm s}^{\rm cri} \sim 0.029$ and $\sim 0.04$ for weakly ($a_{\rm k}\rightarrow 0$) and rapidly ($a_{\rm k}\rightarrow 1$) rotating black hole. Furthermore, we observe that $Φ_{\rm s}^{\rm cri}$ decreases with increasing viscosity ($α$), and shocked accretion solutions continue to exist for $α\lesssim 0.065$ ($a_{\rm k} \rightarrow 0$) and $\lesssim 0.104$ ($a_{\rm k} \rightarrow 1$), respectively.

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arXiv2024

First detection of X-ray polarization in Galactic ULX pulsar Swift J0243.6$+$6124 with {\it IXPE}

We report the results of first ever spectro-polarimetric analyses of the Galactic ultra-luminous X-ray pulsar Swift J0243.6$+6124$ during the 2023 outburst using quasi-simultaneous {\it IXPE}, {\it NICER} and {\it NuSTAR} observations. A pulsation of period $\sim 9.79$ s is detected in {\it IXPE} and {\it NuSTAR} observations with pulse fractions (PFs) $\sim 18\%$ ($2-8$ keV) and $\sim 28\%$ ($3-78$ keV), respectively. Energy dependent study of the pulse profiles with {\it NuSTAR} indicates an increase in PF from $\sim 27\%$ ($3-10$ keV) to $\sim 50\%$ ($40-78$ keV). Further, epoch-dependent polarimetric measurements during the decay phase of the outburst confirm the detection of significant polarization, with the polarization degree (PD) and polarization angle (PA) ranging between $\sim 2 -3.1\%$ and $\sim8.6^{\circ}-10.8^{\circ}$, respectively, in the $2-8$ keV energy range. We also observe that the PD increases up to $\sim 4.8\%$ at higher energies ($\gtrsim 5$ keV) with dominating \texttt{bbodyrad} flux contribution ($1.5 \lesssim F_{\rm BB}/F_{\rm PL} \lesssim 3.4$) in the {\it IXPE} spectra. The phase-resolved polarimetric study yields PD as $\sim 1.7-3.1\%$ suggesting a marginal correlation with the pulse profiles. Moreover, the broad-band ($0.6-70$ keV) energy spectrum of combined {\it NICER} and {\it NuSTAR} observations is well described by the combination of \texttt{bbodyrad} and \texttt{cutoffpl} components with seed photon temperature ($kT_{\rm bb}$) $\sim 0.86 \pm 0.03$ keV and photon index ($Γ$) $\sim 0.98 \pm 0.01$. With the above findings, we infer that the observed `low' PD in Swift J0243.6$+6124$ is attributed possibly due to `vacuum resonance' effect between the overheated and relatively cooler regions of the neutron star boundary layer.

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arXiv2024

Long term wide-band spectro-temporal studies of outbursting black hole candidate sources GX 339-4 and H 1743-322: AstroSat and NuSTAR results

We present a comprehensive spectro-temporal analyses of recurrent outbursting black hole sources GX 339$-$4 and H 1743$-$322 using available AstroSat and NuSTAR archival observations during 2016-2024. The nature of the outburst profiles of both sources are examined using long-term MAXI/GSC and Swift/BAT lightcurves, and failed as well as successful outbursts are classified. Wide-band (0.5-60 keV) spectral modelling with disc (diskbb) and Comptonized (Nthcomp) components indicates that GX 339-4 transits from hard ($kT_{bb}=0.12-0.77$ keV, $Γ_{nth}=1.54-1.74$ and $L_{bol}=0.91-11.56$\% $L_{Edd}$) to soft state ($kT_{in}~[\approx{kT}_{bb}]=0.82-0.88$ keV, $Γ_{nth}=1.46-3.26$, $L_{bol}=19.59-30.06\%L_{Edd}$) via intermediate state ($kT_{in}~[\approx{kT}_{bb}]=0.56-0.88$ keV, $Γ_{nth}=1.76-2.66$, $L_{bol}=2.90-16.09\%L_{Edd}$), whereas H 1743-322 transits from quiescent to hard state ($Γ_{nth}=1.57-1.71$, $L_{bol}=2.08-3.48\%L_{Edd}$). We observe type-B and type-C Quasi-periodic Oscillations (QPOs) in GX 339-4 with increasing frequencies ($0.10-5.37$ Hz) along with harmonics. For H 1743-322, prominent type-C QPOs are observed in frequency range 0.22-1.01 Hz along with distinct harmonics. Energy-dependent power spectral studies reveal that fundamental QPO and harmonics disappear beyond 20 keV in GX 339-4, whereas fundamental QPO in H 1743-322 persists up to 40 keV. We also observe that type-C $QPO_{rms}\%$ decreases with energy for both sources although such variations appear marginal for type-B QPOs. Additionally, we report non-monotonic behavior of photon index with plasma temperature and detection of annihilation line. Finally, we discuss the relevance of the observational findings in the context of accretion dynamics around black hole binaries.

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arXiv2024

Low angular momentum general relativistic magnetohydrodynamic accretion flow around rotating black holes with shocks

We investigate the global structure of general relativistic magneto-hydrodynamic (GRMHD) accretion flows around Kerr black holes containing shock waves, where the disk is threaded by radial and toroidal magnetic fields. We self-consistently solve the GRMHD equations that govern the flow motion inside the disk and for the first time to our knowledge, we obtain the shock-induced global GRMHD accretion solutions around weakly as well as rapidly rotating black holes for a set of fundamental flow parameters, such as energy ($E$), angular momentum ($L$), radial magnetic flux ($Φ$), and iso-rotation parameter ($F$). We show that shock properties, namely shock radius ($r_{\rm sh}$), compression ratio ($R$) and shock strength ($Ψ$) strongly depends on $E$, $L$, $Φ$, and $F$. We observe that shock in GRMHD flow continues to exist for wide range of the flow parameters, which allows us to identify the effective domain of parameter space in $L-E$ plane where shock solutions are feasible. Moreover, we examine the modification of the shock parameter space and find that it shifts towards the lower angular momentum values with increasing $Φ$ and black hole spin ($a_{\rm k}$). Finally, we compute the critical radial magnetic flux ($Φ^{\rm cri}$) that admits shocks in GRMHD flow and ascertain that $Φ^{\rm cri}$ is higher (lower) for black hole of spin $a_{\rm k} = 0.99$ ($0.0$) and vice versa.

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arXiv2024

Study of relativistic hot accretion flow around Kerr-like Wormhole

We investigate the structure of relativistic, low-angular momentum, inviscid advective accretion flow in a stationary axisymmetric Kerr-like wormhole (WH) spacetime, characterized by the spin parameter ($a_{\rm k}$), the dimensionless parameter ($β$), and the source mass ($M_{\rm WH}$). In doing so, we self-consistently solve the set of governing equations describing the relativistic accretion flow around a Kerr-like WH in the steady state, and for the first time, we obtain all possible classes of global accretion solutions for transonic as well as subsonic flows. We study the properties of dynamical and thermodynamical flow variables and examine how the nature of the accretion solutions alters due to the change of the model parameters, namely energy ($\mathcal{E}$), angular momentum ($λ$), $a_{\rm k}$, and $β$. Further, we separate the parameter space in $λ-\mathcal{E}$ plane according to the nature of the flow solutions, and study the modification of the parameter space by varying $a_{\rm k}$ and $β$. Moreover, we retrace the parameter space in $a_{\rm k}-β$ plane that allows accretion solutions containing multiple critical points. Finally, we calculate the disc luminosity ($L$) considering free-free emissions for transonic solutions as these solutions are astrophysically relevant and discuss the implication of this model formalism in the context of astrophysical applications.

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arXiv2024

Accretion flows around spinning compact objects in the post-Newtonian regime

We present the structure of a low angular momentum accretion flows around rotating compact objects incorporating relativistic corrections up to the leading post-Newtonian order. To begin with, we formulate the governing post-Newtonian hydrodynamic equations for the mass and energy-momentum flux without imposing any symmetries. However, for the sake of simplicity, we consider the flow to be stationary, axisymmetric, and inviscid. Toward this, we adapt the polytropic equation of state (EoS) and analyze the vertically integrated accretion flow confined to the equatorial plane. It is shown that the spin-orbit effects manifest themselves in the accretion dynamics. In the present analysis, we focus on global transonic accretion solutions, where a subsonic flow enters far away from the compact object and gradually gains radial velocity as it moves inwards. Thus, the flow becomes supersonic after reaching a certain radius, known as the critical point. To better understand the transonic solutions and examine the effect of post-Newtonian corrections, we classify the post-Newtonian equations into semi-relativistic (SR), semi-Newtonian (SN), and non-relativistic (NR) limits and compare the accretion solutions and their corresponding flow variables. With these, we find that SR and SN flow are in good agreement all throughout, although they deviate largely from the NR ones. Interestingly, the density profile seems to follow the profile $ρ\propto r^{-3/2}$ in the post-Newtonian regime. The present study has the potential to connect Newtonian and GR descriptions of accretion dynamics.

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arXiv2024

Discovery of evolving low-frequency QPOs in hard X-rays ($\sim 100$ keV) observed in black hole Swift J1727.8-1613 with $AstroSat$

We report the first detection of evolving Low-Frequency Quasi-periodic Oscillation (LFQPO) frequencies in hard X-rays upto $100$ keV with $AstroSat/LAXPC$ during `unusual' outburst phase of Swift J1727.8-1613 in hard-intermediate state (HIMS). The observed LFQPO in $20 - 100$ keV has a centroid $ν_{_{\rm QPO}}=1.43$ Hz, a coherence factor $Q= 7.14$ and an amplitude ${\rm rms_{_{\rm QPO}}} = 10.95\%$ with significance $σ= 5.46$. Type-C QPOs ($1.09-2.6$ Hz) are found to evolve monotonically during HIMS of the outburst with clear detection in hard X-rays ($80 - 100$ keV), where ${\rm rms_{_{\rm QPO}}}$ decreases ($\sim 12-3\%$) with energy. Further, $ν_{_{\rm QPO}}$ is seen to correlate (anti-correlate) with low (high) energy flux in $2-20$ keV ($15-50$ keV). Wide-band ($0.7 - 40$ keV) energy spectrum of $NICER/XTI$ and $AstroSat/LAXPC$ is satisfactorily described by the `dominant' thermal Comptonization contribution ($\sim 88\%$) in presence of a `weak' signature of disk emissions ($kT_{\rm in} \sim 0.36$ keV) indicating the harder spectral distribution. Considering source mass $M_{\rm BH}=10M_\odot$ and distance $1.5 < {\rm d~(kpc)} < 5$, the unabsorbed bolometric luminosity is estimated as $\sim 0.03-0.92\%L_{\rm Edd}$. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.

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arXiv2024

Study of mass outflow rates from magnetized advective accretion disk around rotating black holes

We develop and discuss a model formalism to study the properties of mass outflows that are emerged out from a relativistic, magnetized, viscous, advective accretion flow around a rotating black hole. In doing so, we consider the toroidal component as the dominant magnetic fields and synchrotron process is the dominant cooling mechanism inside the accretion disk. With this, we self-consistently solve the coupled accretion-ejection governing equations in the steady state and obtain the shock-induced global inflow-outflow solutions in terms of the inflow parameters, namely plasma-$β$ ($=p_{\rm gas}/p_{\rm mag}$, $p_{\rm gas}$ and $p_{\rm mag}$ being gas and magnetic pressures), accretion rates ($\dot m$) and viscosity ($α_{\rm B}$), respectively. Using these solutions, we compute the mass outflow rate ($R_{\dot m}$, the ratio of outflow to inflow mass flux) and find that mass loss from the magnetized accretion disk continues to take place for wide range of inflow parameters and black hole spin ($a_{\rm k}$). We also observe that $R_{\dot m}$ strongly depends on plasma-$β$, $\dot m$, $α_{\rm B}$ and $a_{\rm k}$, and it increases as the magnetic activity inside the accretion disk is increased. Further, we compute the maximum mass outflow rate ($R^{\rm max}_{\dot m}$) by freely varying the inflow parameters and find that for magnetic pressure dominated disk, $R^{\rm max}_{\dot m} \sim 24\%$ ($\sim 30\%$) for $a_{\rm k}=0.0$ ($0.99$). Finally, while discussing the implication of our model formalism, we compute the maximum jet kinetic power using $R^{\rm max}_{\dot m}$ which appears to be in close agreement with the observed jet kinetic power of several black hole sources.

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arXiv2024

Study of relativistic accretion flow in the $f(R)$ theory of gravity

We present the properties of relativistic, inviscid, low angular momentum, advective accretion flow in a $f(R)$ gravity theory that satisfactorily mimics the asymptotically flat vacuum solutions of the Einstein's equations. With this, we solve the governing equations describing the accretion flow and obtain the global transonic accretion solutions in terms of flow energy (${\cal E}$), angular momentum ($λ$) and gravity parameter ($A$) that determines the effect of $f(R)$ gravity. We observe that depending on the model parameters, flow may contain either single or multiple critical points. We separate the effective domain of the parameter space in $λ-{\cal E}$ plane that admits accretion solutions possessing multiple critical points and observe that solution of this kind continues to form for wide range of the flow parameters. We examine the modification of the parameter space and reveal that it gradually shrinks with the decrease of $A$, and ultimately disappears for $A=-2.34$. Finally, we calculate the disk luminosity ($L$) considering bremsstrahlung emission process and find that global accretion solutions passing through the inner critical point are more luminous compared to the outer critical point solutions.

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arXiv2024

Accretion flow in deformed Kerr spacetime: Spectral energy distributions from free-free emission

In this paper, we study the properties of accretion flow including its spectral features in Johannsen and Psaltis (JP) non-Kerr spacetime. In doing so, we numerically solve the governing equations that describe the flow motion around the compact objects in a general relativistic framework, where spin ($a_{k}$) and deformation parameters ($\varepsilon$) demonstrate the nature of the central source, namely black hole (BH) or naked singularity (NS). With this, we obtain all possible classes of global accretion solutions ($i. e.$, O, A, W and I-type) by varying the energy ($E$) and angular momentum ($λ$) of the relativistic accretion flow, and examine the role of thermal bremsstrahlung emission in studying the spectral energy distributions (SEDs) of the accretion disc. We divide the parameter space in $λ-E$ plane in terms of the different classes of accretion solutions for BH and NS models. We further calculate the disc luminosity ($L$) corresponding to these accretion solutions, and observe that I-type solutions yield higher $L$ and SEDs than the remaining types of solutions for both BH and NS models. For BH model, SEDs for W and I-type solutions differ significantly from the results for O and A-type solutions for low $E$ values. On the contrary, for NS model, SEDs for different accretion solutions are identical in the whole parameter space of $λ$ and $E$. We also examine the effect of $\varepsilon$ on the SEDs and observe that a non-Kerr BH yields higher SEDs than the usual Kerr BH. Finally, for accretion solutions of identical $E$ and $λ$, we compare the SEDs obtained from BH and NS models, and find that naked singularity objects produce more luminous power spectra than the black holes.

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arXiv2023

Properties of relativistic hot accretion flow around rotating black hole with radially varying viscosity

We examine the effect of variable viscosity parameter ($α$) in relativistic, low angular momentum advective accretion flow around rotating black holes. Following the recent simulation studies of magnetohydrodynamic disk that reveal the radial variation of $α(r)$, we theoretically investigate the properties of the global transonic accretion flow considering a one-dimensional power law prescription of viscosity parameter as $α(r) \propto r^θ$, where the viscosity exponent $θ$ is a constant. In doing so, we adopt the relativistic equation of state and solve the fluid equations that govern the flow motion inside the disk. We find that depending on the flow parameters, accretion flow experiences centrifugally supported shock transition and such shocked accretion solutions continue to exist for wide ranges of the flow energy, angular momentum, accretion rate and viscosity exponent, respectively. Due to shock compression, the hot and dense post-shock flow (hereafter PSC) can produce the high energy radiations after reprocessing the soft photons from the pre-shock flow via inverse Comptonization. Since PSC is usually described using shock radius ($r_s$), compression ratio ($R$) and shock strength ($S$), we study the role of $θ$ in deciding $r_s$, $R$ and $S$, respectively. Moreover, we obtain the parameter space for shock and find that possibility of shock formation diminishes as $θ$ is increased. Finally, we compute the limiting value of $θ$ ($i.e., θ^{\rm max}$) that admits shock and find that flow can sustain more viscosity when it accretes onto rapidly rotating ($a_{\rm k} \rightarrow 1$) black hole in comparison to weakly rotating ($a_{\rm k} \rightarrow 0$) black hole.

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arXiv2023

First detection of X-ray polarization in thermal state of LMC X-3: Spectro-polarimetric study with IXPE

We report a comprehensive spectro-polarimetric study of the black hole binary LMC X$-3$ using simultaneous {\it IXPE}, {\it NICER} and {\it NuSTAR} observations in 0.5$-$20 keV energy band. The broad-band energy spectrum (0.5$-$20 keV) with {\it NICER} and {\it NuSTAR} is well described by the disc emission of temperature $\sim$1.1 keV and a weak Comptonizing tail beyond $\sim$10 keV. This evidently suggests a disc-dominated spectral state of the source with disc contribution of $\sim$96\%. The lack of variability ($rms$$\sim$0.5\%) in the power spectrum further corroborates the high/soft nature of the source. A significant polarization degree (PD) of 3.04$\pm$0.40\% (>7$σ$) at a polarization angle (PA) of $-$44.24$^{\circ}$$\pm$3.77$^{\circ}$ (>7$σ$) is found in 2$-$8 keV energy range of {\it IXPE}. In addition, PD is seen to increase with energy up to $\sim$4.35$\pm$0.98\% (>3$σ$) in 4$-$8 keV band. Further, we attempt to constrain the source spin ($a_{*}$) using broad-band spectral modelling that indicates a weakly rotating `hole' in LMC X$-3$ with $a_{*}=0.273_{-0.012}^{+0.011}-0.295_{-0.021}^{+0.008}$ ($90\%$ confidence). Based on the spectro-polarimetric results, we infer that the polarization in LMC X$-3$ is resulted possibly due to the combined effects of the direct and/or reflected emissions from a partially ionized disc atmosphere. Finally, we discuss the relevance of our findings.

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arXiv2023

Unveiling the accretion scenario of BH-ULXs using XMM-Newton observations

We present a comprehensive spectro-temporal analysis of five ultraluminous X-ray sources (ULXs) with central object likely being a black hole, using archival {\it XMM-Newton} observations. These sources, namely NGC 1313 X-1, NGC 5408 X-1, NGC 6946 X-1, M82 X-1 and IC 342 X-1, reveal short-term variability with fractional variance of $1.42-27.28\%$ and exhibit QPOs with frequency $ν_{\rm QPO} \sim 8-667$ mHz. Long-term evolution of ULXs energy spectra ($0.3 - 10$ keV; excluding M82 X$-$1) are described satisfactorily with a model combination that comprises a thermal Comptonization component (\texttt{nthComp}, yielding $Γ_{\rm nth} \sim 1.48 - 2.65$, $kT_{\rm e} \sim 1.62 - 3.76$ keV, $τ\sim 8 - 20$, y-par $\sim 1.16 - 6.24$) along with a standard disc component (\texttt{diskbb}, $kT_{\rm in} \sim 0.16 - 0.54$ keV). We find that these ULXs generally demonstrate anti-correlation between disc luminosity and inner disc temperature as $L_{\rm disc} \propto T_{\rm in}^α$, where $α= - 3.58 \pm 0.04$ for NGC 1313 X-1 and IC 342 X-1, $α= - 8.93 \pm 0.11$ for NGC 6946 X-1, and $α= - 10.31 \pm 0.10$ for NGC 5408 X-1. We also obtain a linear correlation between bolometric luminosity $L_{\rm bol}$ and $Γ_{\rm nth}$ that indicates spectral softening of the sources when $L_{\rm bol}$ increases. We observe that in presence of QPO, Comptonized seed photon fraction varies in between $\sim 5 - 20 \%$, while the Comptonized flux contribution ($50 - 90\%$) dominates over disc flux. Utilizing $ν_{\rm QPO}$ and $L_{\rm bol}$, we constrain ULXs mass by varying their spin ($a_{\rm k}$) and accretion rate ($\dot m$). We find that NGC 6946 X-1 and NGC 5408 X-1 seem to accrete at sub-Eddington accretion rate provided their central sources are rapidly rotating, whereas IC 342 X-1 and NGC 1313 X-1 can accrete in sub/super-Eddington limit irrespective to their spin values.

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arXiv2023

Global Transonic Solution of Hot Accretion Flow with Thermal Conduction

We examine the effect of thermal conduction on the low-angular momentum hot accretion flow (HAF) around non-rotating black holes accreting mass at very low rate. While doing so, we adopt the conductive heat flux in the saturated form, and solve the set of dynamical equations corresponding to a steady, axisymmetric, viscous, advective accretion flow using numerical methods. We study the dynamical and thermodynamical properties of accreting matter in terms of the input parameters, namely energy ($\varepsilon_0$), angular momentum ($\ell_0$), viscosity parameter ($α$), and saturation constant ($Φ_{\rm s}$) regulating the effect of thermal conduction. We find that $Φ_{\rm s}$ plays a pivotal role in deciding the transonic properties of the global accretion solutions. In general, when $Φ_{\rm s}$ is increased, the critical point ($r_{\rm c}$) is receded away from the black hole, and flow variables are altered particularly in the outer part of the disc. To quantify the physically acceptable range of $Φ_{\rm s}$, we compare the global transonic solutions with the self-similar solutions, and observe that the maximum saturation constant ($Φ^{\rm max}_{\rm s}$) estimated from the global solutions exceeds the saturated thermal conduction limit ($Φ_{\rm sc}$) derived from the self-similar formalism. Moreover, we calculate the correlation between $α$ and $Φ^{\rm max}_{\rm s}$ and find ample disagreement between global solutions and self-similar solutions. Further, using the global flow variables, we compute the Bernoulli parameter ($Be$) which remains positive all throughout the disc, although flow becomes loosely unbound for higher $Φ_{\rm s}$. Finally, we indicate the relevance of this work in the astrophysical context in explaining the possibility of massloss/outflows from the unbound disc.

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arXiv2022

Properties of accretion flow in deformed Kerr spacetime

We study the properties of a low-angular momentum, inviscid, advective accretion flow in a deformed Kerr spacetime under the framework of general theory of relativity. We solve the governing equations that describe the flow motion in terms of input parameters, namely energy ($E$), angular momentum ($λ$), spin ($a_{\rm k}$) and deformation parameter ($\varepsilon$), respectively. We find that global transonic accretion solutions continue to exist in non-Kerr spacetime. Depending on the input parameters, accretion flow is seen to experience shock transition and we find that shocked induced accretion solutions are available for a wide range of the parameter space in $λ-E$ plane. We examine the modification of the shock parameter space with $\varepsilon$, and find that as $\varepsilon$ is increased, the effective region of the parameter space is reduced, and gradually shifted towards the higher $λ$ and lower $E$ domain. In addition, for the first time in the literature, we notice that accretion flow having zero angular momentum admits shock transition when spacetime deformation is significantly large. Interestingly, beyond a critical limit of $\varepsilon^{\rm max}$, the nature of the central object alters from black hole (BH) to naked singularity (NS) and we identify $\varepsilon^{\rm max}$ as function of $a_{\rm k}$. Further, we examine the accretion solutions and its properties around the naked singularity as well. Finally, we indicate the implications of the present formalism in the context of astrophysical applications.

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arXiv2022

Study of relativistic accretion flow around KTN black hole with shocks

We present the global solutions of low angular momentum, inviscid, advective accretion flow around Kerr-Taub-NUT (KTN) black hole in presence and absence of shock waves. These solutions are obtained by solving the governing equations that describe the relativistic accretion flow in KTN spacetime which is characterized by the Kerr parameter ($a_{\rm k}$) and NUT parameter ($n$). During accretion, rotating flow experiences centrifugal barrier that eventually triggers the discontinuous shock transition provided the relativistic shock conditions are satisfied. In reality, the viability of shocked accretion solution appears more generic over the shock free solution as the former possesses high entropy content at the inner edge of the disc. Due to shock compression, the post-shock flow (equivalently post-shock corona, hereafter PSC) becomes hot and dense, and therefore, can produce high energy radiations after reprocessing the soft photons from the pre-shock flow via inverse Comptonization. In general, PSC is characterized by the shock properties, namely shock location ($r_s$), compression ratio ($R$) and shock strength ($S$), and we examine their dependencies on the energy (${\cal E}$) and angular momentum ($λ$) of the flow as well as black hole parameters. We identify the effective domain of the parameter space in $λ-{\cal E}$ plane for shock and observe that shock continues to form for wide range of flow parameters. We also find that $a_{\rm k}$ and $n$ act oppositely in determining the shock properties and shock parameter space. Finally, we calculate the disc luminosity ($L$) considering free-free emissions and observe that accretion flows containing shocks are more luminous compared to the shock free solutions.

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