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Sandip K. Chakrabarti

Publications and source records attributed to Sandip K. Chakrabarti.

At least 235 records · Page 13Linked to original sources
arXiv1997

Bondi Flows on Compact Objects Revisited

We revisit Bondi flows on black holes, naked singularities and neutron stars keeping in mind recent progress in understanding of the flow behaviour near compact objects. In an adiabatic spherical accretion on black holes and naked singularities, standing shock waves do not form, but shocks on neutron stars may or may not form depending on the boundary conditions. For low accretion rates (hard state), these objects may behave similarly. However, for high accretion rates (soft state), the converging flow on a black hole will produce a weak hard tail of energy spectral slope of $\sim 1.5$ (apart from the soft bump) while the flow on a neutron star or a naked singularity will have no hard tail. The soft bumps of neutron star candidates may generally be at higher energies than those of naked singularities.

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arXiv1997

Spectral Properties of Accretion Disks Around Black Holes II -- Sub-Keplerian Flows With and Without Shocks

Close to a black hole, the density of the sub-Keplerian accreting matter becomes higher compared to a spherical flow due to the presence of a centrifugal barrier independent of whether or not a standing shock actually forms. This hot dense flow intercepts soft photons from a cold Keplerian disk and reprocesses them to form high energy X-rays and gamma rays. We study the spectral properties of various models of accretion disks where a Keplerian disk on the equatorial plane may or may not be flanked by a sub-Keplerian disk and the sub-Keplerian flow may or may not possess standing shocks. From comparison with the spectra, we believe that the observed properties could be explained better when both the components (Keplerian and sub-Keplerian) are simultaneously present close to a black hole, even though the sub-Keplerian halo component may have been produced out of the Keplerian disk itself at larger radii. We are able to understand soft and hard states of black hole candidates, properties of X-ray novae outbursts, and quasi-periodic oscillations of black hole candidates using these two component models. We fit spectra of X-ray novae GS1124-68 and GS2000+25 and satisfactorily reproduce the light curves of these objects.

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arXiv1997

Recent Progresses Of Accretion Disk Models Around Black Holes

Accretion disk models have evolved from Bondi flows in the 1950s to Keplerian disks in the 1970s and finally to advective transonic flows in the 1990s. We discuss recent progresses in this subject and show that sub-Keplerian flows play a major role in determining the spectral properties of black holes. Centrifugal pressure supported enhanced density region outside the black hole horizon produces hard X-rays and gamma rays by reprocessing intercepted soft photons emitted by the Keplerian disk terminated farther out from the black holes. Quasi-periodic oscillations can also be understood from the dynamic or thermal resonance effects of the enhanced density region.

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arXiv1997

Accretion Processes Around Black Holes And Neutron Stars: Advective Disk Paradigm

We review models which include advective accretion disks onto compact objects and discuss the influence of the centrifugal pressure supported high density region close to the compact objects on the emitted spectra. We show that the stationary and non-stationary spectral properties (such as low and high states, quasi-periodic oscillations, quiescent and rising phases of X-ray novae, etc.) of black hole candidates could be satisfactorily explained by the advective disk models.

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arXiv1997

Numerical Simulations of Rotating Accretion Flows near a Black Hole

We present time-dependent solutions of thin, supersonic accretion flows near a black hole and compare them with analytical solutions. Such flows of inviscid, adiabatic gas are characterized by the specific angular momentum and the specific energy. We confirm that for a wide range of above parameters a stable standing shock wave with a vortex inside it forms close to the black hole. Apart from steady state solutions, we show the existence of non-steady solutions for thin accretion flows where the accretion shock is destroyed and re-generated periodically. The unstable behavior should be caused by dynamically induced instabilities, since inviscid, adiabatic gas is considered. We discuss possible relevance of the periodic behavior on quasi-periodic oscillations (QPOs) observed in galactic and extragalactic black hole candidates.

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arXiv1996

Numerical Simulations Of Advective Flows Around Black Holes

We present fully time-dependent solutions of accretion processes on black holes and compare them with analytical solutions. We use Smoothed Particle Hydrodynamics and Total Variation Diminishing mathods as the numerical tenchniques. Apart from steady state solutions, we also obtain time dependent results where the shock oscillates with the characteristic period of quasi-periodic oscillations (QPOs) observed in galactic and extragalactic black hole candidates. This oscillation produces 10-15% variation in the hard X-rays in galactic candidates and in soft X-rays in Extragalactic candidates.

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arXiv1996

Unified Accretion Disk Models Around Black Holes and Neutron Stars and Their Spectral Properties

We review the current understanding of accretion flows around compact objects with a special emphasis on advective disks. We discuss the influence of the centrifugal pressure supported high density region around compact objects (where shocks may also form) on the emitted spectra. We show that the stationary and non-stationary spectral properties (such as, low and high states, transition of states, quasi-periodic oscillations, quiescent and rising phases of X-ray novae, etc.) of both low mass and supermassive black hole candidates could be satisfactorily explained within the framework of the analytical solution of the advective disks without invoking any ad hoc components such as Compton clouds or magnetic corona.

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arXiv1996

Aspects of Accretion Processes On a Rotating Black Hole

We describe the most general nature of accretion and wind flows around a compact object and emphasize on the properties which are special to black hole accretion. The angular momentum distribution in the most general solution is far from Keplerian, and the non-Keplerian disks can include standing shock waves. We also present fully time dependent numerical simulation results to show that they agree with these analytical solutions. We describe the spectral properties of these accretion disks and show that the soft and hard states of the black hole candidates could be explained by the change of the accretion rate of the disk. We present fits of the observational data to demonstrate the presence of sub-Keplerian flows around black holes.

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arXiv1996

New Twists In The Study Of Gravity Wave Emission In Systems With Massive Black Holes

Traditionlly, gravitational wave emission from a coalescing binary system is computed using point mass approximations without considering any accretion disk. However, it is believed that in many of the galactic nuclei, there are supermassive central black holes surrounded by accretion disks. These accretion disks must be necessarily supersonic on and outside the horizon simply because the radial velocity (in the corotating frame) has to be the velocity of light on the horizon while the sound speed must be smaller. However, supersonic flows are typically sub-Keplerian. Thus, smaller black holes and neutron stars (which are on instantaneously Keplerian orbit and lose energy and angular momentum through gravitational radiation) on their way to coalesce with the central black hole must accrete negative angular momentum from the disk. We study here the way these disks affect the gravitational wave emission.

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arXiv1996

Global Solutions of Viscous Transonic Flows in Kerr Geometry I: Weak Viscosity Limit

We present fully general relativistic equations governing viscous transonic flows in vertical equilibrium in Kerr geometry. We find the complete set of global solutions (both for Optically thick and optically thin flows) in the weak viscosity limit. We show that for a large region of parameter space, centrifugal pressure supported standing shocks can form in accretion and winds very close to the black hole horizon, both for co-rotating and contra-rotating flows. We compute the nature of the shear tensor for complete transonic solutions and discuss the consequences of its reversal properties.

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arXiv1996

Zero Energy Rotating Accretion Flows near a Black Hole

We characterize the nature of thin, axisymmetric, inviscid, accretion flows of cold adiabatic gas with zero specific energy in the vicinity of a black hole by the specific angular momentum. Using two-dimensional hydrodynamic simulations in cylindrical geometry, we present various regimes in which the accretion flows behave distinctly differently. When the flow has a small angular momentum $(λ\lsimλ_b)$, most of the material is accreted into the black hole forming a quasi-spherical flow or a simple disk-like structure around it. When the flow has a large angular momentum (typically, larger than the marginally bound value, $λ\gsimλ_{mb}$), almost no accretion into the black hole occurs. Instead, the flow produces a stable standing shock with one or more vortices behind it and is deflected away at the shock as a conical outgoing wind of higher entropy. If the flow has an angular momentum somewhat smaller than $λ_{mb}$ $(λ_{u}\lsimλ\lsimλ_{mb})$, a fraction (typically, $5-10$\%) of the incoming material is accreted into the black hole, but the the flow structure formed is similar to that as for $λ\gsimλ_{mb}$. Some of the deflected material is accreted back into the black hole, while the rest is blown away as an outgoing wind. These two cases with $λ\gsimλ_u$ correspond those studied in the previous works by Molteni, Lanzafame, \& Chakrabarti (1994) and Ryu \etal (1995). However, the flow with an angular momentum close to the marginally stable value $(λ_{ms})$ is found to be unstable. More specifically, if $λ_b\lsimλ\simλ_{ms}\lsimλ_u$, the flow displays a distinct periodicity in the sense that the inner part of the disk is built and

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arXiv1996

Grand Unification of Solutions of Accretion and Winds around Black Holes and Neutron Stars

We provide the complete set of global solutions of viscous transonic flows (VTFs) around black holes and neutron stars. These solutions describe the optically thick and optically thin flows from the horizon of the black hole or from the neutron star surface to the location where the flow joins with a Keplerian disk. We study the nature of the multiple sonic points as functions of advection, rotation, viscosity, heating and cooling. Stable shock waves, which join two transonic solutions, are found to be present in a large region of the parameter space. We classify the solutions in terms of whether or not the flow can have a standing shock wave. We find no new topology of solutions other than what are observed in our previous studies of isothermal VTFs. We particularly stress the importance of the boundary conditions and argue that we have the most complete solution of accretion and winds around black holes and neutron stars.

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arXiv1996

Numerical Simulations of Standing Shocks in Accretion Flows around Black Holes: A Comparative Study

We compare the results of numerical simulations of thin and quasi-spherical (thick) accretion flows with existing analytical solutions. We use a Lagrangian code based on the Smooth Particle Hydrodynamics (SPH) scheme and an Eulerian finite difference code based on the Total Variation Diminishing (TVD) scheme. In one-dimensional thin flows, the results of the simulations, with or without shocks, agree very well with each other and with analytical solutions. In two-dimensional thick flows, the general features, namely the locations and strengths of centrifugal and turbulent pressure supported shocks, centrifugal barriers, and the funnel walls which are expected from analytical models, agree very well, though the details vary. Generally speaking, the locations of the shocks may be better obtained by SPH since the angular momentum is strictly preserved in SPH, but the shocks themselves are better resolved by TVD. The agreement of these code test results with analytical solutions provides us with confidence to apply these codes to more complex problems which we will discuss elsewhere.

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arXiv1996

Accretion Processes On a Black Hole

We describe astrophysical processes around a black hole keeping primarily the physics of accretion in mind. In Section 1, we briefly discuss the formation, evolution and detection of black holes. We also discuss the difference of flow properties around a black hole and a Newtonian star. In Section 2, we present past and present developments in the study of spherically accreting flows. We study the properties of Bondi flow with and without radiative transfer. In the presence of significant angular momentum, which is especially true in a binary system, matter will be accreted as a thin Keplerian disk. In Section 3, we discuss a large number of models of these disks including the more popular standard disk model. We present magnetized disk models as well. Since the angular momentum is high in these systems, rotational motion is the most dominant component compared to the radial or the vertical velocity components. In Section 4, we study thick disk models which are of low angular momentum but still have no significant radial motion. The accretion rates could be very high causing the flow to become radiation dominated and the disk to be geometrically thick. For low accretion rates, ion pressure supported disks are formed. In Section 5, we extensively discuss the properties of transonic flows which has with sub-Keplerian angular momentum. In the absence of shock discontinuities, these sub-Keplerian flows are basically advecting, similar to Bondi flows, close to the black holes, though far away they match Keplerian or sub-Keplerian disks. In presence of shocks, the post-shock flow becomes rotation dominated similar to thick disks. In Section 6, we present results of important numerical simulations of accretion flows. Significant results from the studies of evolution of viscous transonic flows are reported. In Section 7, we discuss some observational evidences of the black hole accretion. We also present a detailed model of a generalized accretion disk and present its spectra and compare with observations. In Section 8, we summarize the review and make concluding remarks.

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arXiv1996

Gravitational Wave Emission From a Binary Black Hole System in Presence of an Accretion Disk

We study time evolution and gravitational wave emission properties of a black hole orbiting {\it inside} an accretion disk surrounding a massive black hole. We simultaneously solve the structure equations of the accretion disk in presence of heating, cooling and viscosity as well as the equations governing the companion orbit. The deviation from Keplerian distribution of angular momentum of the disk due to pressure and advection effects causes a significant exchange of angular momentum between the disk and the companion. This significantly affects the gravitational wave emission properties from the binary system. We show that when the companion is light, the effect is extremely important and must be taken into account while interpreting gravitational wave signals from such systems.

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arXiv1996

Spectral Properties of Accretion Disks Around Galactic and Extragalactic Black Holes

We study the spectral properties of a very general class of accretion disks which can be decomposed into three distinct components apart from a shock at $r=r_s$: (1) An optically thick Keplerian disk on the equatorial plane ($r>r_s$), (2) A sub-Keplerian optically thin halo above and below this Keplerian disk $r>r_s$ and (3) A hot, optically slim, $τ\sim 1$ postshock region $r<r_s\sim 5-10 r_g$ where $r_g$ is the Schwarzschild radius. The postshock region intercepts soft photons from the Keplerian component and reradiates them as hard X-rays and $γ$ rays after Comptonization. We solve two-temperature equations in the postshock region with Coulomb energy exchange between protons and electrons, and incorporating radiative processes such as bremsstrahlung and Comptonization. We also present the exact prescription to compute the reflection of the hard X-rays from the cool disk. We produce radiated spectra from both the disk components as functions of the accretion rates and compare them with the spectra of galactic and extragalactic black hole candidates. We find that the transition from hard state to soft state is smoothly initiated by a single parameter, namely, the mass accretion rate of the disk. In the soft state, when the postshock region is very optically thick and cooled down, bulk motion of the converging flow determines the spectral index to be about $1.5$ in agreement with observations.

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arXiv1995

On the Spectral Slopes of Hard X-ray Emission from Black Hole Candidates

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.

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