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Radiatively inefficient accretion flow simulations with cooling: implications for black hole transients

Das, Upasana and Sharma, Prateek (2013) Radiatively inefficient accretion flow simulations with cooling: implications for black hole transients. In: MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 435 (3). pp. 2431-2444.

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Official URL: http://dx.doi.org/10.1093/mnras/stt1452

Abstract

We study the effects of optically thin radiative cooling on the structure of radiatively inefficient accretion flows (RIAFs). The flow structure is geometrically thick, and independent of the gas density and cooling, if the cooling time is longer than the viscous time-scale (i.e. t(cool) greater than or similar to t(visc)). For higher densities, the gas can cool before it can accrete and forms the standard geometrically thin, optically thick Shakura-Sunyaev disc. For usual cooling processes (such as bremsstrahlung), we expect an inner hot flow and an outer thin disc. For a short cooling time the accretion flow separates into two phases: a radiatively inefficient hot coronal phase and a cold thin disc. We argue that there is an upper limit on the density of the hot corona corresponding to a critical value of t(cool)/t(ff)( similar to 10-100), the ratio of the cooling time and the free-fall time. Based on our simulations, we have developed a model for transients observed in black hole X-ray binaries (XRBs). An XRB in a quiescent hot RIAF state can transition to a cold blackbody-dominated state because of an increase in the mass accretion rate. The transition from a thin disc to a RIAF happens because of mass exhaustion due to accretion; the transition happens when the cooling time becomes longer than the viscous time at inner radii. Since the viscous time-scale for a geometrically thin disc is quite long, the high-soft state is expected to be long-lived. The different time-scales in black hole transients correspond to different physical processes such as viscous evolution, cooling and free fall. Our model captures the overall features of observed state transitions in XRBs.

Item Type: Journal Article
Additional Information: copyright for this article belongs to Authors
Keywords: accretion; accretion discs; X-rays: binaries
Department/Centre: Division of Physical & Mathematical Sciences > Joint Astronomy Programme
Division of Physical & Mathematical Sciences > Physics
Depositing User: Id for Latest eprints
Date Deposited: 13 Nov 2013 10:48
Last Modified: 13 Nov 2013 10:48
URI: http://eprints.iisc.ac.in/id/eprint/47739

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