Santabrata Das | Calcutta University (original) (raw)

Papers by Santabrata Das

Research paper thumbnail of Study of shocks in relativistic viscous accretion flow around Kerr black hole

We study the relativistic viscous accretion flows around the Kerr black holes. We present governi... more We study the relativistic viscous accretion flows around the Kerr black holes. We present governing equations that describe the flow motion in full general relativity and solve them to obtain the complete set of global transonic solutions in terms of the flow parameters, namely energy ( E), angular momentum ( L) and viscosity (α). We obtain a new type of accretion solution which was not reported earlier. Further, to the best of our knowledge, we show for the first time that viscous accretion solutions may contain shock waves particularly when flow simultaneously passes through both inner critical point (r_ in) and outer critical point (r_ out) before entering to the Kerr black hole. We examine the shock properties, namely shock location (r_s) and compression ratio (R, measure of density compression across the shock front) and show that shock can form for a large region of parameter space in L- E plane. We study the effect of viscous dissipation on the shock parameter space and find ...

Research paper thumbnail of Dynamically induced shock oscillation in the accretion disc around black holes

We investigate the global accretion solution around a stati on ry black hole using the smooth par... more We investigate the global accretion solution around a stati on ry black hole using the smooth particle hydrodynamics (SPH) me thod. With the suitable choice of the input parameters, accretion flow u ndergoes shock transition that are oscillatory in nature. Such unstable so lutions exhibit the modulation of the inner part of the disk, such as post-shock c rona (PSC) and perhaps be responsible as the source of Quasi Periodic Os cillations (QPOs) of emergent high energy radiation commonly observed in black hole sources.

Research paper thumbnail of Study of relativistic accretion flow in Kerr-Taub-NUT spacetime

Physical Review D, 2020

We study the properties of the relativistic, steady, axisymmetric, low angular momentum, inviscid... more We study the properties of the relativistic, steady, axisymmetric, low angular momentum, inviscid, advective, geometrically thin accretion flow in a Kerr-Taub-NUT (KTN) spacetime which is characterized by the Kerr parameter ($a_{\rm k}$) and NUT parameter ($n$). Depending on armka_{\rm k}armk and nnn values, KTN spacetime represents either a black or a naked singularity. We solve the governing equations that describe the relativistic accretion flow in KTN spacetime and obtain all possible global transonic accretion solutions around KTN black hole in terms of the energy (calE)({\cal E})(calE) and angular momentum (lambda)(\lambda)(lambda) of the flow. We identify the region of the parameter space in lambda−calE\lambda-{\cal E}lambda−calE plane that admits the flow to possess multiple critical points for KTN black hole. We examine the modification of the parameter space due to armka_{\rm k}armk and nnn and find that the role of armka_{\rm k}armk and nnn in determining the parameter space is opposite to each other. This clearly indicates that the...

Research paper thumbnail of AstroSat view of GRS 1915+105 during the soft state: detection of HFQPOs and estimation of mass and spin

Monthly Notices of the Royal Astronomical Society

We report the results of AstroSat observations of GRS 1915+105 obtained using 100 ks Guaranteed T... more We report the results of AstroSat observations of GRS 1915+105 obtained using 100 ks Guaranteed Time during the soft state. The colour–colour diagram indicates a variability class of δ with the detection of high-frequency quasi-periodic oscillation (HFQPO) in the power density spectra. The HFQPO is seen to vary in the frequency range of 67.96–70.62 Hz with percentage rms ∼0.83–1.90 per cent and significance varying from 1.63 to 7.75. The energy dependent power spectra show that the HFQPO features are dominant only in 6–25 keV energy band. The broad-band energy spectra (0.7–50 keV) of Soft X-ray Telescope and Large Area X-ray Proportional Counter modelled with nthComp and powerlaw imply that the source has an extended corona in addition to a compact ‘Comptonizing corona’ that produces high-energy emission and exhibits HFQPOs. The broad-band spectral modelling indicates that the source spectra are well described by thermal Comptonization with electron temperature (kTe) of 2.07–2.43 ke...

Research paper thumbnail of Standing shocks in magnetized advection accretion flows onto a rotating black hole

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Limitations of the pseudo-Newtonian approach in studying the accretion flow around a Kerr black hole

Research paper thumbnail of Shocks in relativistic viscous accretion flows around Kerr black holes

Monthly Notices of the Royal Astronomical Society

We study the relativistic viscous accretion flows around the Kerr black holes. We present the gov... more We study the relativistic viscous accretion flows around the Kerr black holes. We present the governing equations that describe the steady state flow motion in full general relativity and solve them in 1.5D to obtain the complete set of global transonic solutions in terms of the flow parameters, namely specific energy (${\cal E}$), specific angular momentum (${\cal L}$) and viscosity (α). We obtain a new type of accretion solution which was not reported earlier. Further, we show for the first time to the best of our knowledge that viscous accretion solutions may contain shock waves particularly when flow simultaneously passes through both inner critical point (rin) and outer critical point (rout) before entering into the Kerr black holes. We examine the shock properties, namely shock location (rs) and compression ratio (R, the measure of density compression across the shock front) and show that shock can form for a large region of parameter space in calL−calE{\cal L}-{\cal E}calL−calE plane. We stu...

Research paper thumbnail of A comparative study of single-temperature and two-temperature accretion flows around black holes

Journal of Astrophysics and Astronomy

Research paper thumbnail of Accretion-ejection in rotating black holes: a model for ‘outliers’ track of radio-X-ray correlation in X-ray binaries

Astrophysics and Space Science

Research paper thumbnail of Properties of two-temperature dissipative accretion flow around black holes

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Properties of magnetically supported dissipative accretion flow around black holes with cooling effects

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Low angular momentum relativistic hot accretion flow around Kerr black holes with variable adiabatic index

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Accretion flow dynamics during 1999 outburst of XTE J1859+226—modeling of broadband spectra and constraining the source mass

Astrophysics and Space Science

Research paper thumbnail of Estimation of mass outflow rates from dissipative accretion disc around rotating black holes

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Analytical Investigation of Mass Outflow Rate in Quasars

Iau Joint Discussion, 2003

ABSTRACT Centrifugal force of rapidly infalling matter onto a quasar brakes the flow and forms a ... more ABSTRACT Centrifugal force of rapidly infalling matter onto a quasar brakes the flow and forms a centrifugal pressure dominated boundary layer (CENBOL) around the central engine. The thermal and magnetic pressure pushes matter along the axis to form observed radio jets. We present analytical work showing how the inflow influences the nature of CENBOL and dictates the outflow rate from the accretion disks. We show that the outflow rate directly depends on the temperature of CENBOL.

Research paper thumbnail of A study of the Correlation of Arrival Directions of UHECRs with the Large Scale Structure of the Universe

Eprint Arxiv 0906 4850, Jun 26, 2009

Ultrahigh energy cosmic rays (UHECRs) are believed to originate from astrophysical sources, which... more Ultrahigh energy cosmic rays (UHECRs) are believed to originate from astrophysical sources, which should trace the large scale structure (LSS) of the universe. On the other hand, the magnetic field in the intergalactic space (IGMF), which also traces the LSS of the universe, deflects the trajectories of the charged UHECRs and spoils the positional correlation of the observed UHECR events with their true sources. To explore this problem, we studied a simulation of the propagation of UHE protons through the magnetized LSS of the universe, reported earlier in Das et al. (2008), in which the IGMF was estimated based on a turbulence dynamo model (Ryu et al. 2008). Hypothetical sources were placed inside clusters and groups of galaxies in the simulated universe, while observers were located inside groups of galaxies that have similar properties as the Local Group. We calculated the statistics of the angular distance between the arrival directions of simulated UHE proton events and the positions of candidate sources in our simulation. We compared the statistics from our simulation with those calculated with the Auger data. We discussed the implication of our works on the nature of the sources of UHECRs.

Research paper thumbnail of Formation of non-steady outflows and QPOs around black hole

We study the time dependent properties of sub-Keplerian viscous accretion flow around the black h... more We study the time dependent properties of sub-Keplerian viscous accretion flow around the black holes. We find that rotating matter feels centrifugal barrier on the way towards the black holes and eventually, shock transition is triggered allowing a part of the post-shock matter to eject out as bipolar outflow. This outflowing matters are supposed to be the precursor of relativistic jets. As viscosity is increased, shock becomes unsteady and start to oscillate when viscosity reached its critical value. This causes the inner part of the disk unsteady resulting periodic ejection of matter from the post-shock region. Also, the same hot and dense post-shock matter emits high energy radiation which modulates quasi-periodically. The power density spectra confirms this features as most of the power is concentrated at a narrow frequency range --- a characteristics (i. e. Quasi-Periodic Oscillation) commonly seen in several outbursting black hole candidates.

Research paper thumbnail of Analytical studies of standing shocks in accretion flows around compact objects

Bulletin of the Astronomical Society of India, Aug 30, 2005

Shocks in advective flows around black holes have become es- sential ingredients in explaining wi... more Shocks in advective flows around black holes have become es- sential ingredients in explaining wide variety of observed features. We first analytically study the properties of shocks in the special case when viscosity and other dissipative processes are negligible. We compare our analytical re- sults with those obtained numerically. The agreement is generally good. In the course of development of

Research paper thumbnail of Unstable mass-outflows in geometrically thick accretion flows around black holes

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Mass-loss from advective accretion disc around rotating black holes

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Study of shocks in relativistic viscous accretion flow around Kerr black hole

We study the relativistic viscous accretion flows around the Kerr black holes. We present governi... more We study the relativistic viscous accretion flows around the Kerr black holes. We present governing equations that describe the flow motion in full general relativity and solve them to obtain the complete set of global transonic solutions in terms of the flow parameters, namely energy ( E), angular momentum ( L) and viscosity (α). We obtain a new type of accretion solution which was not reported earlier. Further, to the best of our knowledge, we show for the first time that viscous accretion solutions may contain shock waves particularly when flow simultaneously passes through both inner critical point (r_ in) and outer critical point (r_ out) before entering to the Kerr black hole. We examine the shock properties, namely shock location (r_s) and compression ratio (R, measure of density compression across the shock front) and show that shock can form for a large region of parameter space in L- E plane. We study the effect of viscous dissipation on the shock parameter space and find ...

Research paper thumbnail of Dynamically induced shock oscillation in the accretion disc around black holes

We investigate the global accretion solution around a stati on ry black hole using the smooth par... more We investigate the global accretion solution around a stati on ry black hole using the smooth particle hydrodynamics (SPH) me thod. With the suitable choice of the input parameters, accretion flow u ndergoes shock transition that are oscillatory in nature. Such unstable so lutions exhibit the modulation of the inner part of the disk, such as post-shock c rona (PSC) and perhaps be responsible as the source of Quasi Periodic Os cillations (QPOs) of emergent high energy radiation commonly observed in black hole sources.

Research paper thumbnail of Study of relativistic accretion flow in Kerr-Taub-NUT spacetime

Physical Review D, 2020

We study the properties of the relativistic, steady, axisymmetric, low angular momentum, inviscid... more We study the properties of the relativistic, steady, axisymmetric, low angular momentum, inviscid, advective, geometrically thin accretion flow in a Kerr-Taub-NUT (KTN) spacetime which is characterized by the Kerr parameter ($a_{\rm k}$) and NUT parameter ($n$). Depending on armka_{\rm k}armk and nnn values, KTN spacetime represents either a black or a naked singularity. We solve the governing equations that describe the relativistic accretion flow in KTN spacetime and obtain all possible global transonic accretion solutions around KTN black hole in terms of the energy (calE)({\cal E})(calE) and angular momentum (lambda)(\lambda)(lambda) of the flow. We identify the region of the parameter space in lambda−calE\lambda-{\cal E}lambda−calE plane that admits the flow to possess multiple critical points for KTN black hole. We examine the modification of the parameter space due to armka_{\rm k}armk and nnn and find that the role of armka_{\rm k}armk and nnn in determining the parameter space is opposite to each other. This clearly indicates that the...

Research paper thumbnail of AstroSat view of GRS 1915+105 during the soft state: detection of HFQPOs and estimation of mass and spin

Monthly Notices of the Royal Astronomical Society

We report the results of AstroSat observations of GRS 1915+105 obtained using 100 ks Guaranteed T... more We report the results of AstroSat observations of GRS 1915+105 obtained using 100 ks Guaranteed Time during the soft state. The colour–colour diagram indicates a variability class of δ with the detection of high-frequency quasi-periodic oscillation (HFQPO) in the power density spectra. The HFQPO is seen to vary in the frequency range of 67.96–70.62 Hz with percentage rms ∼0.83–1.90 per cent and significance varying from 1.63 to 7.75. The energy dependent power spectra show that the HFQPO features are dominant only in 6–25 keV energy band. The broad-band energy spectra (0.7–50 keV) of Soft X-ray Telescope and Large Area X-ray Proportional Counter modelled with nthComp and powerlaw imply that the source has an extended corona in addition to a compact ‘Comptonizing corona’ that produces high-energy emission and exhibits HFQPOs. The broad-band spectral modelling indicates that the source spectra are well described by thermal Comptonization with electron temperature (kTe) of 2.07–2.43 ke...

Research paper thumbnail of Standing shocks in magnetized advection accretion flows onto a rotating black hole

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Limitations of the pseudo-Newtonian approach in studying the accretion flow around a Kerr black hole

Research paper thumbnail of Shocks in relativistic viscous accretion flows around Kerr black holes

Monthly Notices of the Royal Astronomical Society

We study the relativistic viscous accretion flows around the Kerr black holes. We present the gov... more We study the relativistic viscous accretion flows around the Kerr black holes. We present the governing equations that describe the steady state flow motion in full general relativity and solve them in 1.5D to obtain the complete set of global transonic solutions in terms of the flow parameters, namely specific energy (${\cal E}$), specific angular momentum (${\cal L}$) and viscosity (α). We obtain a new type of accretion solution which was not reported earlier. Further, we show for the first time to the best of our knowledge that viscous accretion solutions may contain shock waves particularly when flow simultaneously passes through both inner critical point (rin) and outer critical point (rout) before entering into the Kerr black holes. We examine the shock properties, namely shock location (rs) and compression ratio (R, the measure of density compression across the shock front) and show that shock can form for a large region of parameter space in calL−calE{\cal L}-{\cal E}calL−calE plane. We stu...

Research paper thumbnail of A comparative study of single-temperature and two-temperature accretion flows around black holes

Journal of Astrophysics and Astronomy

Research paper thumbnail of Accretion-ejection in rotating black holes: a model for ‘outliers’ track of radio-X-ray correlation in X-ray binaries

Astrophysics and Space Science

Research paper thumbnail of Properties of two-temperature dissipative accretion flow around black holes

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Properties of magnetically supported dissipative accretion flow around black holes with cooling effects

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Low angular momentum relativistic hot accretion flow around Kerr black holes with variable adiabatic index

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Accretion flow dynamics during 1999 outburst of XTE J1859+226—modeling of broadband spectra and constraining the source mass

Astrophysics and Space Science

Research paper thumbnail of Estimation of mass outflow rates from dissipative accretion disc around rotating black holes

Monthly Notices of the Royal Astronomical Society

Research paper thumbnail of Analytical Investigation of Mass Outflow Rate in Quasars

Iau Joint Discussion, 2003

ABSTRACT Centrifugal force of rapidly infalling matter onto a quasar brakes the flow and forms a ... more ABSTRACT Centrifugal force of rapidly infalling matter onto a quasar brakes the flow and forms a centrifugal pressure dominated boundary layer (CENBOL) around the central engine. The thermal and magnetic pressure pushes matter along the axis to form observed radio jets. We present analytical work showing how the inflow influences the nature of CENBOL and dictates the outflow rate from the accretion disks. We show that the outflow rate directly depends on the temperature of CENBOL.

Research paper thumbnail of A study of the Correlation of Arrival Directions of UHECRs with the Large Scale Structure of the Universe

Eprint Arxiv 0906 4850, Jun 26, 2009

Ultrahigh energy cosmic rays (UHECRs) are believed to originate from astrophysical sources, which... more Ultrahigh energy cosmic rays (UHECRs) are believed to originate from astrophysical sources, which should trace the large scale structure (LSS) of the universe. On the other hand, the magnetic field in the intergalactic space (IGMF), which also traces the LSS of the universe, deflects the trajectories of the charged UHECRs and spoils the positional correlation of the observed UHECR events with their true sources. To explore this problem, we studied a simulation of the propagation of UHE protons through the magnetized LSS of the universe, reported earlier in Das et al. (2008), in which the IGMF was estimated based on a turbulence dynamo model (Ryu et al. 2008). Hypothetical sources were placed inside clusters and groups of galaxies in the simulated universe, while observers were located inside groups of galaxies that have similar properties as the Local Group. We calculated the statistics of the angular distance between the arrival directions of simulated UHE proton events and the positions of candidate sources in our simulation. We compared the statistics from our simulation with those calculated with the Auger data. We discussed the implication of our works on the nature of the sources of UHECRs.

Research paper thumbnail of Formation of non-steady outflows and QPOs around black hole

We study the time dependent properties of sub-Keplerian viscous accretion flow around the black h... more We study the time dependent properties of sub-Keplerian viscous accretion flow around the black holes. We find that rotating matter feels centrifugal barrier on the way towards the black holes and eventually, shock transition is triggered allowing a part of the post-shock matter to eject out as bipolar outflow. This outflowing matters are supposed to be the precursor of relativistic jets. As viscosity is increased, shock becomes unsteady and start to oscillate when viscosity reached its critical value. This causes the inner part of the disk unsteady resulting periodic ejection of matter from the post-shock region. Also, the same hot and dense post-shock matter emits high energy radiation which modulates quasi-periodically. The power density spectra confirms this features as most of the power is concentrated at a narrow frequency range --- a characteristics (i. e. Quasi-Periodic Oscillation) commonly seen in several outbursting black hole candidates.

Research paper thumbnail of Analytical studies of standing shocks in accretion flows around compact objects

Bulletin of the Astronomical Society of India, Aug 30, 2005

Shocks in advective flows around black holes have become es- sential ingredients in explaining wi... more Shocks in advective flows around black holes have become es- sential ingredients in explaining wide variety of observed features. We first analytically study the properties of shocks in the special case when viscosity and other dissipative processes are negligible. We compare our analytical re- sults with those obtained numerically. The agreement is generally good. In the course of development of

Research paper thumbnail of Unstable mass-outflows in geometrically thick accretion flows around black holes

Monthly Notices of the Royal Astronomical Society, 2015

Research paper thumbnail of Mass-loss from advective accretion disc around rotating black holes

Monthly Notices of the Royal Astronomical Society, 2015