Two-temperature accretion around rotating black holes: a description of the general advective flow paradigm in the presence of various cooling processes to explain low to high luminous sources

S R Rajesh,Banibrata Mukhopadhyay

doi:10.1111/j.1365-2966.2009.15925.x

Abstract

We investigate the viscous two temperature accretion discs around rotating black holes. We describe the global solution of accretion flows with a sub-Keplerian angular momentum profile, by solving the underlying conservation equations including explicit cooling processes selfconsistently. Bremsstrahlung, synchrotron and inverse Comptonization of soft photons are considered as possible cooling mechanisms, for sub-Eddington, Eddington and super-Eddington mass accretion rates around Schwarzschild and Kerr black holes with a Kerr parameter 0.998. It is found that the flow, during its infall from the Keplerian to sub-Keplerian transition region to the black hole event horizon, passes through various phases of advection -- general advective paradigm to radiatively inefficient phase and vice versa. Hence the flow governs much lower electron temperature ~10^8-10^{9.5} K, in the range of accretion rate in Eddington units 0.01 <~ \mdot <~ 100, compared to the hot protons of temperature ~ 10^{10.2} - 10^{11.8}K. Therefore, the solution may potentially explain the hard X-rays and \gamma-rays emitted from AGNs and X-ray binaries. We then show that a weakly viscous flow is expected to be cooling dominated, particularly at the inner region of the disc, compared to its highly viscous counterpart which is radiatively inefficient. With all the solutions in hand, we finally reproduce the observed luminosities of the under-fed AGNs and quasars (e.g. Sgr A^*) to ultra-luminous X-ray sources (e.g. SS433), at different combinations of input parameters such as mass accretion rate, ratio of specific heats. The set of solutions also predicts appropriately the luminosity observed in the highly luminous AGNs and ultra-luminous quasars (e.g. PKS 0743-67).

Highlights

The cool Keplerian accretion disc (Pringle & Rees 1972; Shakura & Sunyaev 1973; Novikov & Thorne 1973) was found to be inappropriate to explain observed hard X-rays, e.g. from Cyg X-1 (Lightman & Shapiro 1975)
They did not include the effect of electron heating self-consistently into the hydrodynamical equation, and the hydrodynamical quantities do not get coupled to the rate of electron heating
In order to implement our model to explain observed sources, we focus on the under-luminous AGNs and quasars (e.g. Sgr A∗), ultraluminous quasars and highly luminous AGNs (e.g. PKS 0743-67) and ultra-luminous X-ray (ULX) sources (e.g. SS433), when the last items are likely to be the “radiation trapped” accretion discs

Summary

Introduction

The cool Keplerian accretion disc (Pringle & Rees 1972; Shakura & Sunyaev 1973; Novikov & Thorne 1973) was found to be inappropriate to explain observed hard X-rays, e.g. from Cyg X-1 (Lightman & Shapiro 1975). It was argued that secular instability of the cool disc swells the optically thick, radiation dominated region to a hot, optically thin, gas dominated region resulting in hard component of spectrum ∼ 100KeV (Thorne & Price 1975; Shapiro, Lightman & Eardley 1976) This region is strictly of two temperatures with electron and ion temperatures respectively ∼ 109K and ∼ 5 × 1011K which confirms that cool, one temperature, pure Keplerian accretion solution is not unique. On the other hand, Chakrabarti & Titarchuk (1995) and later Mandal & Chakrabarti (2005) modeled two temperature accretion flows around Schwarzschild black holes in the general “advective paradigm”, emphasizing possible formation of shock and its consequences therein They did not include the effect of electron heating self-consistently into the hydrodynamical equation, and the hydrodynamical quantities do not get coupled to the rate of electron heating (see Rajesh & Mukhopadhyay 2009)

Objectives

Methods

Conclusion