The slimming effect of advection on black-hole accretion flows

J.-P Lasota,M A Abramowicz,A Sadowski,R S S Vieira,R Narayan

doi:10.1051/0004-6361/201527636

J.-P Lasota, M A Abramowicz + Show 3 more

Open Access

https://doi.org/10.1051/0004-6361/201527636

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Abstract

At super-Eddington rates accretion flows onto black holes have been described as slim (aspect ratio $H/R \lesssim 1$) or thick (H/R >1) discs, also known as tori or (Polish) doughnuts. The relation between the two descriptions has never been established, but it was commonly believed that at sufficiently high accretion rates slim discs inflate, becoming thick. We wish to establish under what conditions slim accretion flows become thick. We use analytical equations, numerical 1+1 schemes, and numerical radiative MHD codes to describe and compare various accretion flow models at very high accretion rates.We find that the dominant effect of advection at high accretion rates precludes slim discs becoming thick. At super-Eddington rates accretion flows around black holes can always be considered slim rather than thick.

Highlights

At high accretion rates discs around compact bodies cease to be thin1
In particular the terms corresponding to advection are neglected. Taking these terms into account, as required for consistency, modifies the conclusions about the disc thickness at high accretion rates. This can be clearly seen in the case of advection-dominated accretion flows, known as slim discs in the optically thick case, and as ADAFs when accretion flows are optically thin
Since the inner (R 30 M) regions of super-Eddington accretion flows onto black holes are likely to always be advectiondominated

Summary

Introduction

At high accretion rates discs around compact bodies cease to be thin. When radiation provides the dominant pressure and opacities are mainly due to electron scattering, the thin-disc equations imply that the disc thickness increases linearly with the accretion rate H(R) ∝ M (Shakura & Sunyaev 1973; Frank et al 2002), where H(R) is the disc semi-thickness at the distance R from the centre. The general belief that with increasing accretion rates discs become very thick: they are tori rather than discs Such accretion flows are supposed to be described adequately only by 2D or 3D structures, contrary to thin discs whose properties (including the observed ones) are depicted very well by a (1 + 1)D formalism. This conclusion might be not self-consistent because it follows from the thin-disc equations in which H/R 1 is assumed and O(H/R) terms omitted. In particular the (proportional to the radial velocity) terms corresponding to advection are neglected Taking these terms into account, as required for consistency, modifies the conclusions about the disc thickness at high accretion rates. This can be clearly seen in the case of advection-dominated accretion flows, known as slim discs in the optically thick case, and as ADAFs when accretion flows are optically thin

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