Journal Article

Effects of Compton cooling on the hydrodynamic and the spectral properties of a two-component accretion flow around a black hole

Himadri Ghosh, Sudip K. Garain, Kinsuk Giri and Sandip K. Chakrabarti

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 416, issue 2, pages 959-971
Published in print September 2011 | ISSN: 0035-8711
Published online September 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.19090.x
Effects of Compton cooling on the hydrodynamic and the spectral properties of a two-component accretion flow around a black hole

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We carry out a time-dependent numerical simulation where both the hydrodynamics and the radiative transfer are coupled together. 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 show that in presence of an axisymmetric soft-photon source the spherically symmetric Bondi flow loses its symmetry and becomes axisymmetric. The low angular momentum flow was observed to slow down close to the axis and formed a centrifugal barrier which added new features into the spectrum. Using the Monte Carlo method, we generated the radiated spectra as functions of the accretion rates. We find that the transitions from a hard state to a soft state is determined by the mass accretion rates of the disc and the halo. We separate out the signature of the bulk motion Comptonization and discuss its significance. We study how the net spectrum is contributed by photons suffering different number of scatterings and spending different amounts of time inside the Compton cloud. We study the directional dependence of the emitted spectrum as well.

Keywords: accretion, accretion discs; black hole physics; hydrodynamics; radiation mechanisms: general; radiative transfer; methods: numerical

Journal Article.  7524 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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