43 results on '"Chakrabarti, Sandip K."'
Search Results
2. Accretion Flow Properties of EXO 1846-031 during Its Multi-peaked Outburst after Long Quiescence.
- Author
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Nath, Sujoy Kumar, Debnath, Dipak, Chatterjee, Kaushik, Bhowmick, Riya, Chang, Hsiang-Kuang, and Chakrabarti, Sandip K.
- Subjects
ACCRETION (Astrophysics) ,X-ray binaries ,BLACK holes - Abstract
We study the recent outburst of the black hole candidate EXO 1846-031, which went into an outburst in 2019 after almost 34 yr in quiescence. We use archival data from the Swift/XRT, MAXI/GSC, NICER/XTI, and NuSTAR/FPM satellites/instruments to study the evolution of the spectral and temporal properties of the source during the outburst. The low-energy (2–10 keV) X-ray flux of the outburst shows multiple peaks, making it a multipeak outburst. Evolving type-C quasi-periodic oscillations are observed in the NICER data in the hard, hard-intermediate, and soft-intermediate states. We use the physical two-component advective flow (TCAF) model to analyze the combined spectra of multiple satellite instruments. According to the TCAF model, the accreting matter is divided into Keplerian and sub-Keplerian parts, and the variation in the observed spectra in different spectral states arises out of the variable contributions of these two types of accreting matter in the total accretion rate. Studying the evolution of the accretion rates and other properties of the accretion flow obtained from the spectral analysis, we show that the multiple peaks in the outburst flux arise out of the variable supply of accreting matter from the pile-up radius. We determine the probable mass of the black hole to be 10.4 − 0.2 + 0.1 M ⊙ from the spectral analysis with the TCAF model. We also estimate the viscous timescale of the source in this outburst to be ∼8 days from the peak difference of the Keplerian and sub-Keplerian mass-accretion rates. [ABSTRACT FROM AUTHOR]
- Published
- 2024
- Full Text
- View/download PDF
3. Spectral signatures of dissipative standing shocks and mass outflow in presence of Comptonization around a black hole
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Mondal, Santanu, Chakrabarti, Sandip K., and Debnath, Dipak
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- 2014
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4. Fast transition between high-soft and low-soft states in GRS 1915 + 105: Evidence for a critically viscous accretion flow
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Naik, S., Rao, A. R., and Chakrabarti, Sandip K.
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- 2002
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5. Interaction of accretion shocks with winds
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Acharya, Kinsuk, Chakrabarti, Sandip K., and Molteni, D.
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- 2002
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6. Radiatively driven winds from effective boundary layer around black holes
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Chattopadhyay, Indranil and Chakrabarti, Sandip K.
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- 2002
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7. Standing shocks around black holes and estimation of outflow rates
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Das, Santabrata and Chakrabarti, Sandip K.
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- 2002
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8. Anomalous outbursts of H 1743-322.
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Ghosh, Arindam and Chakrabarti, Sandip K
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X-ray binaries , *PHOTON counting , *FLUX (Energy) - Abstract
Using soft (1.5–3 keV) and hard (3–12 keV) photon counts of All Sky Monitor (ASM) in Rossi X-ray Timing Explorer (RXTE) satellite, we have proposed recently that there is a significant time lag between the infall time-scales of two components in the Two-Component Advective Flow paradigm, where a standard slow moving Keplerian disc is surrounded by a fast moving halo. The time lag is clearly due to the difference in viscosity in the flow components and the size of the Keplerian disc may be considered to be proportional to this arrival time lag. In this paper, using RXTE /ASM (1.5–12 keV) data, we examine eight successive outbursts of the low-mass X-ray binary H 1743-322 since 2003 from a new perspective. The day-to-day temporal evolution of a dynamic photon index, Θ, as well as its cross-correlation with the soft and hard energy fluxes show that the aforesaid time lag was the longest during the brightest outburst of 2003 – thereby indicating its largest Keplerian disc. The disc size diminished thereafter during subsequent weaker outbursts. Moreover, Θ decides spectral transitions of any outburst. We show from the behaviour of Θ alone that the outburst of October 2008 was anomalous while the outburst of 2003 was twin (anomalous + normal). In fact, each normal outburst was either preceded or followed by an otherwise premature outburst showing different degrees of anomaly. This makes H 1743-322 an enigmatic source and a subject of further study. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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- View/download PDF
9. Implications for accretion flow dynamics from a spectral study of Swift J1357.2–0933.
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Mondal, Santanu and Chakrabarti, Sandip K
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ACCRETION (Astrophysics) , *BLACK holes , *GRAVITATIONAL collapse , *BINARY stars , *BINARY pulsars - Abstract
We report a detailed spectral study of the Swift J1357.2–0933 low-mass X-ray binary during its 2017 outburst using Swift and NuSTAR observations. We fit the data with a two-component advective flow (TCAF) model and a power-law model. We observe that the source is in a hard state during the outburst, where the size of the Compton cloud changes significantly with disc accretion rate. The typical disc accretion rate for this source is |${\sim } 1.5\hbox{--}2.0\,{{\ \rm per\ cent}}$| of the Eddington accretion rate |$(\skew2\dot{M}_\mathrm{Edd})$|. The model-fitted intermediate shock compression ratio gives an indication of the presence of a jet, which is reported in the literature in different energy bands. We also split the NuSTAR data into three equal segments and fit them with the model. We check spectral stability using a colour–colour diagram and an accretion rate ratio (ARR) versus intensity diagram using different segments of the light curve, but do not find any significant variation in the hardness ratio or in the accretion rate ratio. To estimate the mass of the candidate, we use an important characteristic of TCAF that the the model normalization always remains a constant. We found that the mass comes out to be in the range of 4.0–6.8 M⊙. From the model-fitted results, we study the disc geometry and different physical parameters of the flow in each observation. The count rate of the source appears to decay on a time-scale of |${\sim } 45\, \mathrm{d}$|. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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10. General relativistic numerical simulation of sub-Keplerian transonic accretion flows on to rotating black holes: Kerr space–time.
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Kim, Jinho, Garain, Sudip K, Chakrabarti, Sandip K, and Balsara, Dinshaw S
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GENERAL relativity (Physics) ,COMPUTER simulation ,TRANSONIC aerodynamics ,ACCRETION (Astrophysics) ,BLACK holes ,DISKS (Astrophysics) - Abstract
We study time evolution of sub-Keplerian transonic accretion flows on to black holes using a general relativistic numerical simulation code. We perform simulations around the black holes having non-zero rotation. We first compare one-dimensional simulation results with theoretical results and validate the performance of our code. Next, we present results of axisymmetric, two-dimensional simulation of advective flows. In the literature, there is no solution which describes steady shock solutions in two dimensions. However, our simulations produce these centrifugal force supported steady shock waves even in presence of strong dragging of inertial frames. Since the post-shock region could be hot and upscatter photons through Comptonization, these shock would put imprints on the spectra. Thus, our solutions, which represent truly new results, could be useful to measure spins through radiation spectrum of accreting Kerr black holes. [ABSTRACT FROM AUTHOR]
- Published
- 2019
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11. Images and spectra of time-dependent two-component advective flow in presence of outflows.
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Chatterjee, Arka, Chakrabarti, Sandip K, Ghosh, Himadri, and Garain, Sudip K
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SEYFERT galaxies , *QUASARS , *ACCRETION (Astrophysics) , *SUPERMASSIVE black holes , *MOLECULAR clouds , *GALAXY spectra - Abstract
Two-component advective flow (TCAF) successfully explains the spectral and temporal properties of outbursting or persistent sources. Images of static TCAF with Compton cloud or centrifugal pressure supported boundary layer due to gravitational bending of photons have been studied before. In this paper, we study time-dependent images of advective flows around a Schwarzschild black hole, which include cooling effects due to Comptonization of soft photons from Keplerian discs as well as the self-consistently produced jets and outflows. We show the overall image of the disc-jet system after convolving with a typical beamwidth. A long exposure image with time-dependent system need not to show the black hole horizon conspicuously, unless one is looking at a soft state with no jet or the system along the jet axis. Assuming these disc-jet configurations are relevant to radio emitting systems also, our results would be useful to look for event horizons in high accretion rate supermassive black holes in Seyfert galaxies, RL Quasars. [ABSTRACT FROM AUTHOR]
- Published
- 2018
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12. Hydrodynamic simulations of accretion flows with time-varying viscosity.
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Roy, Abhishek and Chakrabarti, Sandip K.
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BLACK holes , *SHOCK waves , *HYDRODYNAMICS , *MEASUREMENT of viscosity , *COMPUTER simulation , *PARAMETER estimation - Abstract
X-ray outbursts of stellar-mass black hole candidates are believed to be due to a sudden rise in viscosity, which transports angular momentum efficiently and increases the accretion rates, causing higher X-ray flux. After the viscosity is reduced, the outburst subsides and the object returns back to the pre-outburst quiescence stage. In the absence of a satisfactory understanding of the physical mechanism leading to such a sharp time dependence of viscous processes, we perform numerical simulations where we include the rise and fall of a viscosity parameter at an outer injection grid, assumed to be located at the accumulation radius where matter from the companion is piled up before being released by enhanced viscosity. We use a power-law radial dependence of the viscosity parameter (α ~ rϵ ), but the exponent (ϵ) is allowed to vary with time to mimic a fast rise and decay of the viscosity parameter. Since X-ray spectra of a black hole candidate can be explained by a Keplerian disc component in the presence of a post-shock region of an advective flow, our goal here is also to understand whether the flow configurations required to explain the spectral states of an outbursting source could be obtained by a time-varying viscosity. We present the results of our simulations to prove that low-angular-momentum (sub-Keplerian) advective flows do form a Keplerian disc in the pre-shock region when the viscosity is enhanced, which disappears on a much longer time-scale after the viscosity is withdrawn. From the variation of the Keplerian disc inside an advective halo, we believe that our result, for the first time, is able to simulate the twocomponent advective flow dynamics during an entire X-ray outburst and explain the observed hysteresis effects in the hardness-intensity diagram. [ABSTRACT FROM AUTHOR]
- Published
- 2017
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- View/download PDF
13. Temporal evolution of photon energy emitted from two-component advective flows: origin of time lag.
- Author
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Chatterjee, Arka, Chakrabarti, Sandip K., and Ghosh, Himadri
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BLACK holes , *ACCRETION (Astrophysics) , *ACCRETION disks , *GALACTIC X-ray sources , *MONTE Carlo method - Abstract
X-ray time lag of black hole candidates contains important information regarding the emission geometry. Recently, study of time lags from observational data revealed very intriguing properties. To investigate the real cause of this lag behavior with energy and spectral states, we study photon paths inside a two-component advective flow (TCAF) which appears to be a satisfactory model to explain the spectral and timing properties. We employ the Monte Carlo simulation technique to carry out the Comptonization process. We use a relativistic thick disk in Schwarzschild geometry as the CENtrifugal pressure supported BOundary Layer (CENBOL) which is the Compton cloud. In TCAF, this is the post-shock region of the advective component. Keplerian disk on the equatorial plane which is truncated at the inner edge i.e. at the outer boundary of the CENBOL, acts as the soft photon source. Ray-tracing code is employed to track the photons to a distantly located observer. We compute the cumulative time taken by a photon during Comptonization, reflection and following the curved geometry on the way to the observer. Time lags between various hard and soft bands have been calculated. We study the variation of time lags with accretion rates, CENBOL size and inclination angle. Time lags for different energy channels are plotted for different inclination angles. The general trend of variation of time lag with QPO frequency and energy as observed in satellite data is reproduced. [ABSTRACT FROM AUTHOR]
- Published
- 2017
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14. Dynamics of magnetic flux tubes in an advective flow around a black hole.
- Author
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Deb, Arnab, Giri, Kinsuk, and Chakrabarti, Sandip K.
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BLACK holes ,MAGNETIC flux ,HYDRODYNAMICS ,ACCRETION disks ,ANGULAR momentum (Mechanics) - Abstract
Entangled magnetic fields entering into an accretion flow would very soon be stretched into a dominant toroidal component due to strong differentially rotating motion inside the accretion disc. This is particularly true forweakly viscous, lowangular momentum transonic or advective discs. We study the trajectories of toroidal flux tubes inside a geometrically thick flow that undergoes a centrifugal force supported shock. We also study effects of these flux tubes on the dynamics of the inflow and the outflow. We use a finite difference method (total variation diminishing) for this purpose and specifically focused on whether these flux tubes significantly affect the properties of the outflows such as its collimation and the rate. It is seen that depending upon the cross-sectional radius of the flux tubes that control the drag force, these field lines may move towards the central object or oscillate vertically before eventually escaping out of the funnel wall (pressure zero surfaces) along the vertical direction. A comparison of results obtained with and without flux tubes show these flux tubes could play a pivotal role in collimation and acceleration of jets and outflows. [ABSTRACT FROM AUTHOR]
- Published
- 2017
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15. General relativistic numerical simulation of sub-Keplerian transonic accretion flows on to black holes: Schwarzschild space-time.
- Author
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Jinho Kim, Garain, Sudip K., Balsara, Dinshaw S., and Chakrabarti, Sandip K.
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RELATIVISTIC particles ,COMPUTER simulation ,TRANSONIC aerodynamics ,ACCRETION (Astrophysics) ,BLACK holes ,SCHWARZSCHILD black holes - Abstract
We study time evolution of sub-Keplerian transonic accretion flows on to black holes using a general relativistic numerical simulation code. We perform simulations in Schwarzschild space-time. We first compare one-dimensional simulation results with theoretical results and validate the performance of our code. Next, we present results of axisymmetric, twodimensional simulation of advective flows. We find that even in this case, for which no complete theoretical analysis is present in the literature, steady-state shock formation is possible. [ABSTRACT FROM AUTHOR]
- Published
- 2017
- Full Text
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16. On the Spectral Slopes of Hard X-Ray Emission from Black Hole Candidates
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Titarchuk, Lev, Chakrabarti, Sandip K., and Ebisawa, Ken
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Physics ,Accretion ,Astrophysics::High Energy Astrophysical Phenomena ,Astrophysics (astro-ph) ,Electron rest mass ,FOS: Physical sciences ,Astronomy and Astrophysics ,Astrophysics ,Black hole physics ,X-rays: General ,Radiation ,Power law ,Accretion (astrophysics) ,Radiation mechanisms: Compton and inverse Compton ,Radiative transfer ,Stars: Neutron ,Black hole ,Space and Planetary Science ,Spectral slope ,Thermal - Abstract
Most black hole candidates exhibit characteristic power-law like hard X-ray emission above $\sim$ 10 keV. In the {\em high state}, in which 2 -- 10 keV luminosity is relatively high, the energy index of the hard X-ray emission is usually greater than 1 --- typically $\sim 1.5$. On the other hand, in the {\em low state}, the hard X-ray energy index is 0.3 -- 0.9. In this paper, we suggest that this difference of the hard X-ray spectral slopes may be due to two different Comptonization mechanisms. We propose that, in the high state, the hard component is governed by the Comptonization due to the bulk motion of the almost freely falling (convergent accretion) flow close to the black hole, rather than thermal Comptonization. The spectral slope of the hard component is insensitive to the disk accretion rate governing the soft component, hence is nearly invariant in spite of the soft component variations. The power-law component due to the bulk motion Comptonization has a sharp cut-off at around the electron rest mass energy, which is consistent with high energy observations of the high state. In the low state, the spectrum is formed due to thermal Comptonization of the low-frequency disk radiation by a sub-Keplerian component (possibly undergoing a centrifugally-supported shock) which is originated from the Keplerian disk. In the limit of low disk accretion rate, the power law index is uniquely determined by the mass accretion rate of the sub-Keplerian component., 12 pages of text and Figures (tar compressed postscript file), To appear in PASJ, Feb. 96 (v. 48, No. 1)
- Published
- 1996
17. Numerical simulation of vertical oscillations in an axisymmetric thick accretion flow around a black hole.
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Deb, Arnab, Giri, Kinsuk, and Chakrabarti, Sandip K.
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BLACK holes ,ACCRETION (Astrophysics) ,AXIAL flow ,CENTRIFUGAL force ,COMPUTER simulation ,FINITE difference method - Abstract
We study time evolution of rotating, axisymmetric, two-dimensional inviscid accretion flows around black holes using a grid-based finite difference method. We do not use reflection symmetry on the equatorial plane in order to inspect if the disc along with the centrifugal barrier oscillated vertically. In the inviscid limit, we find that the CENtrifugal pressure supported BOundary Layer (CENBOL) is oscillating vertically, more so, when the specific angular momentum is higher. As a result, the rate of outflow produced from the CENBOL, also oscillates. Indeed, the outflow rates in the upper half and the lower half are found to be anticorrelated. We repeat the exercise for a series of specific angular momentum λ of the flow in order to demonstrate effects of the centrifugal force on this interesting behaviour. We find that, as predicted in theoretical models of discs in vertical equilibrium, the CENBOL is produced only when the centrifugal force is significant and more specifically, when λ > 1.5. Outflow rate itself is found to increase with λ as well and so is the oscillation amplitude. The cause of oscillation appears to be due to the interaction among the back flow from the centrifugal barrier, the outflowing winds and the inflow. For low angular momentum, the back flow as well as the oscillation are missing. To our knowledge, this is the first time that such an oscillating solution is found with a well-tested grid-based finite difference code, and such a solution could be yet another reason of why quasi-periodic oscillations should be observed in black hole candidates that are accreting low angular momentum transonic flows. [ABSTRACT FROM AUTHOR]
- Published
- 2016
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18. Viscosity parameter in dissipative accretion flows with mass outflow around black holes.
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Nagarkoti, Shreeram and Chakrabarti, Sandip K.
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ACCRETION (Astrophysics) , *VISCOSITY , *ENERGY dissipation , *HYDRODYNAMICS , *BLACK holes , *COMPUTER simulation - Abstract
Numerical hydrodynamic simulation of inviscid and viscous flows have shown that significant outflows could be produced from the CENtrifugal pressure-supported BOundary Layer or CENBOL of an advective disc. However, this barrier is weakened in presence of viscosity, more so, if there are explicit energy dissipations at the boundary layer itself. We study effects of viscosity and energy dissipation theoretically on the outflow rate and show that, as the viscosity or energy dissipation (or both) rises, the prospect of formation of outflows is greatly reduced, thereby verifying results obtained through observations and numerical simulations. Indeed, we find that in a dissipative viscous flow, shocks in presence of outflows can be produced only if the Shakura-Sunyaev viscosity parameter α is less than 0.2. This is a direct consequence of modification of the Rankine-Hugoniot relation across the shock in a viscous flow, when the energy dissipation and mass-loss in the form of outflows from the post-shock region are included. If we ignore the effects of mass-loss altogether, the standing dissipative shocks in viscous flows may occur only if α < 0.27. These limits are tighter than the absolute limit of α = 0.3 valid for a situation when the shock itself neither dissipates energy nor any outflow is formed. We compute typical viscosity parameters required to understand spectral and temporal properties of several black hole candidates such as GX399-4, MAXI J1659-152 and MAXI J1836-194 and find that required α are indeed well within our prescribed limit. [ABSTRACT FROM AUTHOR]
- Published
- 2016
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19. Segregation of a Keplerian disc and sub-Keplerian halo from a transonic flow around a black hole by viscosity and cooling processes.
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Giri, Kinsuk, Garain, Sudip K., and Chakrabarti, Sandip K.
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DISKS (Astrophysics) ,TRANSONIC flow ,COOLING ,VISCOSITY ,COMPUTER simulation ,PARAMETER estimation - Abstract
A black hole accretion is necessarily transonic. In presence of sufficiently high viscosity and cooling effects, a low angular momentum transonic flow can become a standard Keplerian disc except close to the hole where it must pass through the inner sonic point. However, if the viscosity is not high everywhere and cooling is not efficient everywhere, the flow cannot completely become a Keplerian disc. In this paper, we show results of rigorous numerical simulations of a transonic flow having vertically varying viscosity parameter (being highest on the equatorial plane) and optical depth dependent cooling processes to showthat the flowindeed segregates into two distinct components as it approaches a black hole. The component on the equatorial plane has properties of a standard Keplerian disc, though the flow is not truncated at the innermost stable circular orbit. This component extends till the horizon as a sub- Keplerian flow. This standard disc is found to be surrounded by a hot, low angular momentum component forming a centrifugal barrier dominated oscillating shock wave, consistent with the Chakrabarti-Titarchuk two-component advective flow configuration. [ABSTRACT FROM AUTHOR]
- Published
- 2015
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20. SEQUENCING THE VARIABILITY CLASSES OF GRS 1915+105.
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PAL, PARTHA SARATHI, CHAKRABARTI, SANDIP K., and NANDI, ANUJ
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ASTROPHYSICS , *PARTICLE physics , *PARTICLE astrophysics , *OPTICAL depth (Astrophysics) , *GALAXIES , *ELECTROMAGNETIC waves , *PHOTONS - Published
- 2012
21. MONTE-CARLO SIMULATIONS OF COMPTONIZATION PROCESS IN A TWO COMPONENT ACCRETION FLOW AROUND A BLACK HOLE IN PRESENCE OF AN OUTFLOW.
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GHOSH, HIMADRI, GARAIN, SUDIP K., GIRI, KINSUK, and CHAKRABARTI, SANDIP K.
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MONTE Carlo method ,BLACK holes ,ASTROPHYSICS ,INTERSTELLAR medium ,ELECTROMAGNETIC waves ,PHOTONS - Published
- 2012
22. Gravitational wave emission from a companion black hole in presence of an accretion disk around a super-massive Kerr black hole.
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Basu, Prasad and Chakrabarti, Sandip K.
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DISKS (Astrophysics) , *ACCRETION (Astrophysics) , *GRAVITY waves , *SUPERMASSIVE black holes , *BINARY stars - Abstract
In this article we investigate the possible effects of the presence of the massive accretion disks on the gravitational wave emission properties of the extreme or intermediate mass ratio inspiralling black hole binaries. We chose a system in which a massive black hole (M = 105 M⊙) is orbiting a central super massive black hole (M = 108 M⊙) endowed with an accretion disk. We show that the drag exerted on the companion by the disk is sufficient to reduce the coalescence time of the binary. We find that the effect of the disk will be significant in reducing the coalescence time and this has to be incorporated to interprete gravitational wave signals emitted from such systems. Same effects should be seen for smaller mass companion also albeit at a smaller scale. [ABSTRACT FROM AUTHOR]
- Published
- 2008
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- View/download PDF
23. EFFECTS OF COMPTON COOLING ON OUTFLOWS IN A TWO COMPONENT ACCRETION FLOW AROUND A BLACK HOLE: RESULTS OF A COUPLED MONTE CARLO-TVD SIMULATION.
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GARAIN, SUDIP K., GHOSH, HIMADRI, and CHAKRABARTI, SANDIP K.
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ACCRETION (Astrophysics) ,COMPTON effect ,BIPOLAR outflows (Astrophysics) ,BLACK holes ,MONTE Carlo method ,HYDRODYNAMICS - Published
- 2015
24. Spectral properties of two-component advective flows with standing shocks in the presence of Comptonization.
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Mondal, Santanu and Chakrabarti, Sandip K.
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SELF-consistent field theory , *ACCRETION disks , *HYDRODYNAMICS , *RADIATIVE transfer , *ACCRETION (Astrophysics) , *BLACK holes - Abstract
We study self-consistently the hydrodynamic and spectral properties of a general class of steady-state accretion discs where we couple both the hydrodynamics and the radiative transfer. We consider a two-component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian disc are reprocessed by the electrons in the halo. We study the transonic properties of such a Comptonized flow. We use the Rankine–Hugoniot relation to obtain the shock locations in the disc and compute the radiated spectrum from this shocked disc. We identify the boundary of the parameter space spanned by the specific energy and angular momentum which allows the formation of the standing shocks. We show how the boundary changes in the presence of Compton cooling. Due to the radiative loss, some energy is removed from the accreting matter and the shock moves towards the black hole to maintain the pressure balance condition. We solve the two-temperature equations with Coulomb energy exchange between the protons and the electrons, and the radiative processes such as the bremsstrahlung and thermal Comptonization. We study the variation of the hydrodynamical and spectral properties as a function of the accretion rates of the Keplerian and sub-Keplerian components. Ours is the most accurate transonic solution of an inviscid flow around a black hole to date. [ABSTRACT FROM AUTHOR]
- Published
- 2013
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25. EVIDENCE OF VARIATION OF THE ACCRETION FLOW GEOMETRY IN GRS 1915 + 105 FROM IXAE AND RXTE DATA.
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PAL, PARTHA SARATHI, CHAKRABARTI, SANDIP K., and NANDI, ANUJ
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ACCRETION (Astrophysics) , *QUASARS , *LIGHT curves , *PHASE transitions , *SPECTRUM analysis , *COMPTON effect , *RADIATIVE transfer , *BLACK holes , *HYDRODYNAMICS - Abstract
The Galactic microquasar GRS 1915 + 105 exhibits various types of light curves. There is, however, no understanding of when a certain type of light curve will be exhibited and only in a handful of cases, the transitions from one type to another have actually been observed. We study the detailed spectral properties in these cases to show that different classes have different ratio of the power-law photon and the blackbody photon. Since the power-law photons are from the Compton cloud, and the intensity of the power-law photon component depends on the degree of interception of the soft photons by the Compton cloud, we conclude that not only the accretion rate, but the accretion flow geometry must also change during a class transition. [ABSTRACT FROM AUTHOR]
- Published
- 2011
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26. FUNDAMENTAL CONCEPTS IN TRANSONIC FLOW PARADIGM OF BLACK HOLE ASTROPHYSICS.
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CHAKRABARTI, SANDIP K.
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BLACK holes , *ASTROPHYSICS , *ACCRETION (Astrophysics) , *TELESCOPES , *ASTRONOMICAL observations , *COMPUTER simulation , *SHOCK waves - Abstract
Exactly three decades ago, it was realized that an accretion flow onto a black hole should be transonic. Since then, the subject has matured considerably and several new and well established concepts and methodologies have replaced earlier ways of studying accretion and winds. Not surprisingly, with the advent of the faster computers as well as better space-based telescopes, the results of numerical simulations and the observations have also improved along with the theory. Today, it is more than satisfying that the results of theory and numerical simulations, even in the context of nonmagnetic flows, agree in details of the observations exceedingly well. I present here several new concepts and intricacies which one has to get familiar with when one talks about the behavior of the transonic flows, either in accretion or in the outflows. [ABSTRACT FROM AUTHOR]
- Published
- 2011
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27. EFFECTS OF THE COMPOSITION ON TRANSONIC PROPERTIES OF ACCRETION FLOWS AROUND BLACK HOLES.
- Author
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CHATTOPADHYAY, INDRANIL and CHAKRABARTI, SANDIP K.
- Subjects
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ACCRETION (Astrophysics) , *BLACK holes , *SHOCK waves , *ANGULAR momentum (Nuclear physics) , *RELATIVISTIC astrophysics , *EQUATIONS of state , *DISKS (Astrophysics) , *HYDRODYNAMICS - Abstract
We study the properties of a steady, multi-species, low angular momentum accretion flow around a Schwarzschild black hole. Each species is described by a relativistic equation of state. We find that the transonic properties depend strongly on the composition of the flow. We find that an electron-positron pair plasma is the least relativistic one. This flow produces only one sonic point very close to the event horizon and does not show multiple critical points for any angular momentum or energy. When the baryons are present, the number of critical points depend on the specific energy content. Since the number of critical points decide whether the flow will have nonlinearities or shock waves, our results imply that whether standing shocks will form or not depends on the flow composition. Thus, for instance, a pure electron-positron pair plasma will never undergo a shock transition, while mixing it with some baryons (common in outflows and jets, for example) as in a completely ionized gas, will have shocks. We study in detail how the baryon loading affects the shock properties and discuss the implications in astrophysical observations. [ABSTRACT FROM AUTHOR]
- Published
- 2011
- Full Text
- View/download PDF
28. ACCRETION PROBLEM IN A KERR BLACK HOLE GEOMETRY VIEWED AS FLOWS IN CONVERGING-DIVERGING DUCTS.
- Author
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CHAKRABARTI, K., MAJUMDAR, M. M., and CHAKRABARTI, SANDIP K.
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ASTROPHYSICS ,GEOMETRY ,AERODYNAMICS ,KERR black holes ,SOLAR wind - Abstract
Accretion flow on a horizon is supersonic, no matter what the flow angular momentum or the spin of the black hole is. This means that a black hole accretion can always be viewed as a flow in a flat space-time through one or more convergent-divergent ducts. In this paper, we study how the area of cross-sections must vary in order that the flow has the same properties in both systems. We show that the accretion flow experiencing a shock is equivalent to having two ducts connected back-to-back, both with a neck where the flow becomes supersonic. We study the pressure and Mach number variations for corotating, contrarotating flows and flows around a black hole with evolving spin. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
29. Evidence for two-component flows around the black hole candidate XTE J1550−564 from spectral features during its 1998–1999 outburst.
- Author
-
Dutta, Broja G. and Chakrabarti, Sandip K.
- Subjects
- *
NUCLEAR counters , *SUPERMASSIVE black holes , *GRAVITATIONAL collapse , *STARS , *GALAXIES , *MILKY Way , *MAGELLANIC clouds , *GALACTIC nuclei , *ASTRONOMY - Abstract
We study the spectral properties of the accretion disc in the Galactic black hole candidate XTE J 564 during the 1998–1999 outburst when the source exhibited double-peaked eruptions. This outburst lasted for 250 d and the 2.5–25.0 keV spectral state varied smoothly from one to another several times. We show that the spectral features of the 1998–1999 outburst could be clearly understood by a two-component (Keplerian and sub-Keplerian) advective flow (TCAF). We concentrate on the spectral data from a Proportional Counter Array instrument on the RXTE satellite for the black hole XTE J 564 and fit them quite satisfactorily using TCAF model. From the spectral fit we calculate the disc parameters, such as the Keplerian rate, the sub-Keplerian rate (halo rate), the shock location and the inner edge of the Keplerian disc. This observation points to the presence of two independent components in the accretion flow and that the accretion rate at all radii need not be constant in an evolving disc. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
30. MONTE CARLO SIMULATIONS OF THE THERMAL COMPTONIZATION PROCESS IN A TWO-COMPONENT ACCRETION FLOW AROUND A BLACK HOLE IN THE PRESENCE OF AN OUTFLOW.
- Author
-
GHOSH, HIMADRI, GARAIN, SUDIP K., CHAKRABARTI, SANDIP K., and LAURENT, PHILIPPE
- Subjects
MONTE Carlo method ,ESTIMATION theory ,EINSTEIN-Podolsky-Rosen experiment ,CENTRIFUGAL force ,ROTATIONAL motion - Abstract
A black hole accretion may have both the Keplerian and the sub-Keplerian component. In the so-called Chakrabarti–Titarchuk scenario, the Keplerian component supplies low-energy (soft) photons while the sub-Keplerian component supplies hot electrons which exchange their energy with the soft photons through Comptonization or inverse Comptonization processes. In the sub-Keplerian component, a shock is generally produced due to the centrifugal force. The postshock region is known as the CENtrifugal pressure–supported BOundary Layer (CENBOL). In this paper, we compute the effects of the thermal and the bulk motion Comptonization on the soft photons emitted from a Keplerian disk by the CENBOL, the preshock sub-Keplerian disk and the outflowing jet. We study the emerging spectrum when the converging inflow and the diverging outflow (generated from the CENBOL) are simultaneously present. From the strength of the shock, we calculate the percentage of matter being carried away by the outflow and determine how the emerging spectrum depends on the outflow rate. The preshock sub-Keplerian flow is also found to Comptonize the soft photons significantly. The interplay between the up-scattering and down-scattering effects determines the effective shape of the emerging spectrum. By simulating several cases with various inflow parameters, we conclude that whether the preshock flow, or the postshock CENBOL or the emerging jet is dominant in shaping the emerging spectrum depends strongly on the geometry of the flow and the strength of the shock in the sub-Keplerian flow. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
31. Studies of dissipative standing shock waves around black holes.
- Author
-
Das, Santabrata, Chakrabarti, Sandip K., and Mondal, Soumen
- Subjects
- *
SUPERMASSIVE black holes , *GRAVITATIONAL collapse , *ANGULAR momentum (Nuclear physics) , *ENERGY dissipation , *ASTRONOMY - Abstract
We investigate the dynamical structure of advective accretion flow around stationary as well as rotating black holes. For a suitable choice of input parameters, such as accretion rate and angular momentum (λ), a global accretion solution may include a shock wave. The post-shock flow is located at a few tens times the Schwarzchild radius and is generally very hot and dense. This successfully mimics the so-called Compton cloud, which is believed to be responsible for emitting hard radiation. Owing to the radiative loss, significant energy is removed from the accreting matter and the shock moves forward towards the black hole in order to maintain the pressure balance across it. We identify the effective area of parameter space that allows accretion flows to have some energy dissipation at the shock . As the dissipation is increased, the parameter space is reduced and finally disappears when the dissipation reaches a critical value. The dissipation has a profound effect on the dynamics of post-shock flow. By moving forward, an unstable shock, the oscillation of which causes quasi-periodic oscillations (QPOs) in the emitted radiation, will produce oscillations of high frequency. Such an evolution of QPOs has been observed in several black hole candidates during their outbursts. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
32. MONTE CARLO SIMULATIONS OF THE THERMAL COMPTONIZATION PROCESS IN A TWO-COMPONENT ACCRETION FLOW AROUND A BLACK HOLE.
- Author
-
GHOSH, HIMADRI, CHAKRABARTI, SANDIP K., and LAURENT, PHILIPPE
- Subjects
- *
MONTE Carlo method , *ACCRETION (Astrophysics) , *SUPERMASSIVE black holes , *PHOTONS , *PARTICLES (Nuclear physics) - Abstract
We compute the effects of thermal Comptonization of soft photons emitted from a Keplerian disk around a black hole by the postshock region of a sub-Keplerian flow, known as the CENtrifugal-pressure-dominated BOundary Layer (CENBOL). We show that the spectral state transitions of black hole candidates could be explained either by varying the outer boundary of the CENBOL, which also happens to be the inner edge of the Keplerian disk, or by changing the central density of the CENBOL, which is governed by the rate of the sub-Keplerian flow. We confirm the conclusions of the previous theoretical studies that the interplay between the intensity of the soft photons emitted by the Keplerian flow, the optical depth and electron temperature of the Comptonizing cloud is responsible for the state transitions in a black hole. [ABSTRACT FROM AUTHOR]
- Published
- 2009
- Full Text
- View/download PDF
33. Dissipative accretion flows around a rotating black hole.
- Author
-
Das, Santabrata and Chakrabarti, Sandip K.
- Subjects
- *
SUPERMASSIVE black holes , *COOLING , *ASTRONOMY , *GRAVITATIONAL collapse , *ACCRETION (Astrophysics) - Abstract
We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely the specific energy , the specific angular momentum (λ) and the accretion rate of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine–Hugoniot shocks are possible is maximum when the flow is dissipation-free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the black hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate , the closer is the shock location. We believe that some of the high-frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
34. Gravitational wave emission from a companion black hole in the presence of an accretion disc around a super-massive Kerr black hole.
- Author
-
Basu, Prasad, Mondal, S., and Chakrabarti, Sandip K.
- Subjects
KERR black holes ,GRAVITY waves ,BINARY stars ,SUPERMASSIVE black holes ,ACCRETION (Astrophysics) - Abstract
Gravitational wave signal characteristics from a binary black hole system in which the companion moves through the accretion disc of the primary are studied. We chose the primary to be a super-massive Kerr black hole and the companion to be a massive black hole to clearly demonstrate the effects. We show that the drag exerted on the companion by the disc is sufficient to reduce the coalescence time of the binary. The drag is primarily due to the fact that the accretion disc on a black hole deviates from a Keplerian disc and becomes sub-Keplerian due to inner boundary condition on the black hole horizon. We consider two types of accretion rates on to the companion. The companion is deeply immersed inside the disc and it can accrete at the Bondi rate which depends on the instantaneous density of the disc. However, an accretion disc can also form around the smaller black hole and it can accrete at its Eddington rate. Thus, this case is also studied and the results are compared. We find that the effect of the disc will be significant in reducing the coalescence time and one needs to incorporate this while interpreting gravitational wave signals emitted from such a binary system. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
35. ACCRETION ONTO COMPACT OBJECTS VIEWED AS A FLOW IN CONVERGING-DIVERGING DUCTS.
- Author
-
CHAKRABARTI, K., MAJUMDAR, M. M., and CHAKRABARTI, SANDIP K.
- Subjects
AERODYNAMICS ,SUPERMASSIVE black holes ,ACCRETION (Astrophysics) ,TRANSONIC aerodynamics ,NOZZLES ,CENTRIFUGAL force - Abstract
Black hole accretion is necessarily transonic and the number of physical sonic points depends on the angular momentum of the flow. We study the properties of such a flow by recasting this idea into an engineering problem in which a flow has a subsonic to supersonic transition when it passes through a de Laval nozzle, i.e. a converging and diverging duct in a flat geometry in the presence of sufficient end pressure difference. Particularly interesting is the case of the centrifugal pressure supported standing shock formation inside an accretion flow, because the flow passes through at least two saddle type sonic points, one before and one after the shock. In this case, the duct itself has two minima and a maximum. We study the properties of such a duct as a function of the inflow parameters and classify all possible types of the flow through this composite nozzle. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
36. NON-LINEARITIES IN ACCRETION AND WINDS AROUND ROTATING BLACK HOLES USING PSEUDO-KERR GEOMETRY.
- Author
-
MONDAL, SOUMEN and CHAKRABARTI, SANDIP K.
- Subjects
- *
SUPERMASSIVE black holes , *ACCRETION (Astrophysics) , *ANGULAR momentum (Mechanics) , *NONLINEAR waves , *SHOCK waves - Abstract
Non-linearities such as shock waves are common in accretion flows around compact objects. Exact quantification of these non-linearities will help testing time-dependent numerical codes. In this paper, we study the detailed properties of these non-linear waves in a steady accretion or wind flows around a rotating black hole. We use a pseudo-Kerr geometry for this purpose. In the context of energy preserving standing shocks, we find that there are two shock locations for a given pair of conserved flow parameters, such as specific energy and angular momentum. We also show that as the Kerr parameter is increased, the shock location moves closer to the black hole. We discuss the astrophysical implications of such solutions. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
37. Quasi-periodic oscillations in quasars to nano-quasars.
- Author
-
Chakrabarti, Sandip K.
- Subjects
- *
OSCILLATIONS , *QUASARS , *SUPERMASSIVE black holes , *ACTIVE galaxies , *SPACE telescopes , *ULTRAVIOLET radiation - Abstract
TAUVEX brings us a unique opportunity to explore the temporal variability of UV emitting objects in the sky. One of the questions that we intend to resolve with TAUVEX is whether the 'variabilities' detected in active galaxies and quasars and in radiations around massive black holes in general are just random variations of the intensities or these are intrinsic to the disk system, and possibly due to the quasi-periodic oscillations (QPOs) which are well known to be observed in smaller black holes (nano-quasars). In this article, we present a physical mechanism for the QPOs and show that this is a generic mechanism which should be manifested in all types of active compact objects, ranging from quasars to nano-quasars. We propose some tests by which we may be able to tell if these are QPOs, even without waiting for a large number of cycles to test the periodicity. We present a few examples to impress that perhaps we have already seen QPOs in some objects. Multi-wavelength observation capabilities in TAUVEX may be used to pinpoint the nature of the variable sources more accurately. [ABSTRACT FROM AUTHOR]
- Published
- 2007
38. Parameter space study of the magnetohydrodynamic accretion flows around compact objects.
- Author
-
Das, Santabrata and Chakrabarti, Sandip K.
- Subjects
- *
ACCRETION (Astrophysics) , *SUPERMASSIVE black holes , *SHOCK waves , *MAGNETOHYDRODYNAMICS , *AXIAL flow , *OSCILLATIONS - Abstract
We solve the magnetohydrodynamic (MHD) equations governing axisymmetric flows around compact objects and found all possible classes of solutions for non-relativistic adiabatic accretion flows. We divide the parameter space in terms of these classes. We study the possibility of the formation of the MHD shock waves and show how the strength of the shocks depends on the flow parameters. We also show regions of the parameter space where the shock conditions are not satisfied and therefore the shocks may oscillate. These solutions are astrophysically interesting as they could give rise to quasi-periodic oscillations seen in hard X-rays. [ABSTRACT FROM AUTHOR]
- Published
- 2007
- Full Text
- View/download PDF
39. Studies of accretion flows around rotating black holes – II. Standing shocks in the pseudo-Kerr geometry.
- Author
-
Mondal, Soumen and Chakrabarti, Sandip K.
- Subjects
- *
ACCRETION (Astrophysics) , *SUPERMASSIVE black holes , *SHOCK waves , *WINDS , *FLUID dynamics - Abstract
Standing, propagating or oscillating shock waves are common in accretion and winds around compact objects. We study the topology of all possible solutions using the pseudo-Kerr geometry. We present the parameter space spanned by the specific energy and angular momentum and compare it with that obtained from the full general relativity to show that the potential can work satisfactorily in fluid dynamics also, provided the polytropic index is suitably modified. We then divide the parameter space depending on the nature of the solution topology. We specifically study the nature of the standing Rankine–Hugoniot shocks. We also show that as the Kerr parameter is increased, the shock location generally moves closer to the black hole. In future, these solutions can be used as guidelines to test numerical simulations around compact objects. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
40. Studies of accretion flows around rotating black holes – I. Particle dynamics in a pseudo-Kerr potential.
- Author
-
Chakrabarti, Sandip K. and Mondal, Soumen
- Subjects
- *
SUPERMASSIVE black holes , *GRAVITATIONAL collapse , *KERR electro-optical effect , *GRAVITATION , *FLUID dynamics - Abstract
In this series of papers, we shall present a simplistic approach to the study of particle dynamics, fluid dynamics and numerical simulations of accretion flows and outflows around rotating black holes. We show that with a suitably modified effective potential of the central gravitating rotating object, one can carry out these studies very accurately. In this approach, one need not use the full general relativistic equations to obtain the salient features of the general relativistic flows provided the Kerr parameter remains within . We present the equatorial and the non-equatorial particle trajectories from our potential and compare salient properties in Kerr and in pseudo-Kerr geometries. Our potential naturally produces accurate results for motions around the Schwarzschild geometry when the black hole angular momentum is set to zero. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
41. SIGNATURES OF ACCRETION SHOCKS IN BROADBAND SPECTRUM OF ADVECTIVE FLOWS AROUND BLACK HOLES.
- Author
-
Mandal, Samir and Chakrabarti, Sandip K.
- Subjects
- *
CENTRIFUGAL force , *SUPERMASSIVE black holes , *GRAVITATIONAL collapse , *ASTRONOMY , *ASTROPHYSICS - Abstract
We compute the effects of the centrifugal pressure supported shock waves on the emitted spectrum from an accretion disk primarily consisting of low angular momentum matter. Electrons are very efficiently accelerated by the accretion shock and acquire power-law distribution. The accelerated particles in turn emit synchrotron radiation in the presence of a stochastic magnetic field in equipartition with the gas. Efficient cooling of the electrons by these soft photons reduces its temperature in comparison to the protons. We explore the nature of the broadband spectra by using Comptonization, bremsstrahlung and synchrotron emission. We then show that there could be two crossing points in a broadband spectrum, one near ~10 keV and the other ~300–400 keV. [ABSTRACT FROM AUTHOR]
- Published
- 2005
- Full Text
- View/download PDF
42. PROPERTIES OF ACCRETION SHOCKS IN VISCOUS FLOWS WITH COOLING EFFECTS.
- Author
-
Das, Santabrata and Chakrabarti, Sandip K.
- Subjects
- *
ACCRETION (Astrophysics) , *SHOCK waves , *VISCOUS flow , *OSCILLATIONS , *ASTROPHYSICS , *PHYSICS research - Abstract
Low angular momentum accretion flows can have standing and oscillating shock waves. We study the region of the parameter space in which multiple sonic points occur in viscous flows in presence of various cooling effects such as bremsstrahlung and Comptonization. We also quantify the parameter space in which shocks are steady or oscillating. We find that cooling induces effects opposite to heating by viscosity even in modifying the topology of the solutions, though one can never be exactly balanced by the other due to their dissimilar dependence on dynamic and thermodynamic parameters. We show that beyond a critical value of cooling, the flow ceases to contain a shock wave. [ABSTRACT FROM AUTHOR]
- Published
- 2004
- Full Text
- View/download PDF
43. Properties of accretion shock waves in viscous flows around black holes.
- Author
-
Chakrabarti, Sandip K. and Das, Santabrata
- Subjects
- *
ACCRETION (Astrophysics) , *SUPERMASSIVE black holes , *SHOCK waves , *VISCOSITY , *ANGULAR momentum (Mechanics) , *OSCILLATIONS - Abstract
Accretion flows having low angular momentum and low viscosity can have standing shock waves. These shocks arise because of the presence of multiple sonic points in the flow. We study the region of the parameter space in which multiple sonic points occur in viscous flows in the absence of cooling. We also separate the parameter space into regions allowing steady shocks and oscillating shocks. We quantify the nature of two critical viscosities which separate the flow topologies. The post-shock region being hotter, it emits harder X-rays and oscillating shocks cause oscillating X-ray intensities giving rise to quasi-periodic oscillations. We show that with the increase in viscosity parameter, the shock always moves closer to the black hole. This implies an enhancement of the quasi-periodic oscillation frequency as viscosity is increased. [ABSTRACT FROM AUTHOR]
- Published
- 2004
- Full Text
- View/download PDF
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