Simulations and Analysis of Gravitationally Redshifted Kerr Black Hole Accretion Discs

Abstract The study of radiation emitted from black hole accretion discs represents a crucial way to understand the main physical properties of these sources, and in particular the black hole spin. Beside spectral analysis, polarimetry is becoming more and more important, motivated by the development of new techniques which will soon allow to perform measurements also in the X- and γ-rays. Photons emitted from black hole accretion discs in the soft state are indeed expected to be polarized, with an energy dependence which can provide an estimate of the black hole spin. Calculations performed so far, however, considered scattering as the only process to determine the polarization state of the emitted radiation, implicitly assuming that the temperatures involved are such that material in the disc is entirely ionized. In this work we generalize the problem by calculating the ionization structure of a surface layer of the disc with the public code cloudy, and then by determining the polarization properties of the emerging radiation using the Monte Carlo code stokes. This allows us to account for absorption effects alongside scattering ones. We show that including absorption can deeply modify the polarization properties of the emerging radiation with respect to what is obtained in the pure-scattering limit. As a general rule, we find that the polarization degree is larger when absorption is more important, which occurs e.g. for low accretion rates and/or spins when the ionization of the matter in the innermost accretion disc regions is far from complete.

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Black hole accretion discs and screened scalar hair

Journal of Cosmology and Astroparticle Physics ◽

10.1088/1475-7516/2016/10/024 ◽

2016 ◽

Vol 2016 (10) ◽

pp. 024-024 ◽

Cited By ~ 8

Author(s):

Anne-Christine Davis ◽

Ruth Gregory ◽

Rahul Jha

Keyword(s):

Black Hole ◽

Accretion Discs ◽

Black Hole Accretion ◽

Scalar Hair ◽

Hole Accretion

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ACCRETION PROBLEM IN A KERR BLACK HOLE GEOMETRY VIEWED AS FLOWS IN CONVERGING–DIVERGING DUCTS

International Journal of Modern Physics D ◽

10.1142/s021827181001827x ◽

2010 ◽

Vol 19 (13) ◽

pp. 2059-2069

Author(s):

K. CHAKRABARTI ◽

M. M. MAJUMDAR ◽

SANDIP K. CHAKRABARTI

Keyword(s):

Black Hole ◽

Angular Momentum ◽

Mach Number ◽

Cross Sections ◽

Kerr Black Hole ◽

Flat Space ◽

Accretion Flow ◽

Black Hole Accretion ◽

Hole Geometry ◽

Hole Accretion

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.

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