Sandip K. Chakrabarti

Bio: Sandip K. Chakrabarti is an academic researcher from Indian Centre for Space Physics. The author has contributed to research in topics: Black hole & Accretion (astrophysics). The author has an hindex of 50, co-authored 477 publications receiving 10411 citations. Previous affiliations of Sandip K. Chakrabarti include Tata Institute of Fundamental Research & Katholieke Universiteit Leuven.

Spectral Properties of Accretion Disks around Galactic and Extragalactic Black Holes

Abstract: 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, $\tau\sim 1$ postshock region $r

Spectral Properties of Accretion Disks Around Galactic and Extragalactic Black Holes

Abstract: 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, $\tau\sim 1$ postshock region $r

Standing Rankine-Hugoniot Shocks in the Hybrid Model Flows of the Black Hole Accretion and Winds

Abstract: This paper presents all the Rankine-Hugoniot shock solutions for the stationary, axially symmetric and rotating adiabatic flows of small transverse thickness in the black hole potential. The recently proposed hybrid model is used to treat the flows which include shocks. It is shown that, for given initial and final states of the flow, there can be as many as four formal shock locations in both the accretion and the winds. Only two (three) of these locations are acceptable for accretion onto black holes (neutron stars), and three of these locations are acceptable for winds. It is shown that the question of the multiplicity of the shocks cannot be resolved by local stability analysis of the shocks. In addition, the properties of the isentropic compression waves and the isothermal shocks are briefly studied. 17 refs.

Resonance Oscillation of Radiative Shock Waves in Accretion Disks around Compact Objects

Abstract: We extend our previous numerical simulation of accretion disks with shock waves when cooling effects are also included. We consider bremsstrahlung and other power law processes: $\Lambda \propto T^{\alpha} \rho^2$ to mimic cooling in our simulation. We employ {\it Smoothed Particle Hydrodynamics} technique as in the past. We observe that for a given angular momentum of the flow, the shock wave undergoes a steady, radial oscillation with the period is roughly equal to the cooling time. Oscillations seem to take place when the disk and cooling parameters (i.e., accretion rate, cooling process) are such that the infall time from shock is of the same order as the post-shock cooling time. The amplitude of oscillation could be up to ten percent of the distance of the shock wave from the black hole when the black hole is accreting. When the accretion is impossible due to the centrifugal barrier, the amplitude variation could be much larger. Due to the oscillation, the energy output from the disk is also seen to vary quasi-periodically. We believe that these oscillations might be responsible for the quasi periodic oscillation (QPO) behaviors seen in several black hole candidates, in neutron star systems as well as dwarf novae outbursts such as SS Cygni and VW Hyi.

A Fundamental Relation Between Supermassive Black Holes and Their Host Galaxies

Abstract: The masses of supermassive black holes correlate almost perfectly with the velocity dispersions of their host bulges, Mbh ∝ σα, where α = 48 ± 05 The relation is much tighter than the relation between Mbh and bulge luminosity, with a scatter no larger than expected on the basis of measurement error alone Black hole masses recently estimated by Magorrian et al lie systematically above the Mbh-σ relation defined by more accurate mass estimates, some by as much as 2 orders of magnitude The tightness of the Mbh-σ relation implies a strong link between black hole formation and the properties of the stellar bulge

First M87 Event Horizon Telescope Results. I. The Shadow of the Supermassive Black Hole

Abstract: When surrounded by a transparent emission region, black holes are expected to reveal a dark shadow caused by gravitational light bending and photon capture at the event horizon. To image and study this phenomenon, we have assembled the Event Horizon Telescope, a global very long baseline interferometry array observing at a wavelength of 1.3 mm. This allows us to reconstruct event-horizon-scale images of the supermassive black hole candidate in the center of the giant elliptical galaxy M87. We have resolved the central compact radio source as an asymmetric bright emission ring with a diameter of 42 +/- 3 mu as, which is circular and encompasses a central depression in brightness with a flux ratio greater than or similar to 10: 1. The emission ring is recovered using different calibration and imaging schemes, with its diameter and width remaining stable over four different observations carried out in different days. Overall, the observed image is consistent with expectations for the shadow of a Kerr black hole as predicted by general relativity. The asymmetry in brightness in the ring can be explained in terms of relativistic beaming of the emission from a plasma rotating close to the speed of light around a black hole. We compare our images to an extensive library of ray-traced general-relativistic magnetohydrodynamic simulations of black holes and derive a central mass of M = (6.5 +/- 0.7) x 10(9) M-circle dot. Our radio-wave observations thus provide powerful evidence for the presence of supermassive black holes in centers of galaxies and as the central engines of active galactic nuclei. They also present a new tool to explore gravity in its most extreme limit and on a mass scale that was so far not accessible.

X-Ray Properties of Black-Hole Binaries

Abstract: We review the properties and behavior of 20 X-ray binaries that contain a dynamically-confirmed black hole, 17 of which are transient systems. During the past decade, many of these transien...

Central masses and broad-line region sizes of active galactic nuclei. ii. a homogeneous analysis of a large reverberation-mapping database

Abstract: We present improved black hole masses for 35 active galactic nuclei (AGNs) based on a complete and consistent reanalysis of broad emission-line reverberation-mapping data From objects with multiple line measurements, we find that the highest precision measure of the virial product cτΔV2/G, where τ is the emission-line lag relative to continuum variations and ΔV is the emission-line width, is obtained by using the cross-correlation function centroid (as opposed to the cross-correlation function peak) for the time delay and the line dispersion (as opposed to FWHM) for the line width and by measuring the line width in the variable part of the spectrum Accurate line-width measurement depends critically on avoiding contaminating features, in particular the narrow components of the emission lines We find that the precision (or random component of the error) of reverberation-based black hole mass measurements is typically around 30%, comparable to the precision attained in measurement of black hole masses in quiescent galaxies by gas or stellar dynamical methods Based on results presented in a companion paper by Onken et al, we provide a zero-point calibration for the reverberation-based black hole mass scale by using the relationship between black hole mass and host-galaxy bulge velocity dispersion The scatter around this relationship implies that the typical systematic uncertainties in reverberation-based black hole masses are smaller than a factor of 3 We present a preliminary version of a mass-luminosity relationship that is much better defined than any previous attempt Scatter about the mass-luminosity relationship for these AGNs appears to be real and could be correlated with either Eddington ratio or object inclination