On the Eddington limit for relativistic accretion discs

TL;DR

This paper investigates the limits of standard relativistic accretion disc models near the Eddington limit, highlighting how relativistic effects, disc thickness, and advection influence the maximum accretion rates and model accuracy.

Contribution

It provides new corrections to the standard accretion disc model accounting for relativistic and thickness effects near the Eddington limit.

Findings

Relativistic corrections can double the local Eddington limit.

Disc thickness and advection effects significantly alter the Eddington limit.

Advection reduces accretion efficiency by several times.

Abstract

Standard accretion disc model relies upon several assumptions, the most important of which is geometrical thinness. Whenever this condition is violated, new physical effects become important such as radial energy advection and mass loss from the disc. These effects are important, for instance, for large mass accretion rates when the disc approaches its local Eddington limit. In this work, we study the upper limits for standard accretion disc approximation and find the corrections to the standard model that should be considered in any model aiming on reproducing the transition to super-Eddington accretion regime. First, we find that for thin accretion disc, taking into account relativistic corrections allows to increase the local Eddington limit by about a factor of two due to stronger gravity in General Relativity (GR). However, violation of the local Eddington limit also means large disc thickness. To consider consequently the disc thickness effects, one should make assumptions upon the two-dimensional rotation law of the disc. For rotation frequency constant on cylinders $r\sin\theta=const$, vertical gravity becomes stronger with height on spheres of constant radius. On the other hand, effects of radial flux advection increase the flux density in the inner parts of the disc and lower the Eddington limit. In general, the effects connected to disc thickness tend to increase the local Eddington limit even more. The efficiency of accretion is however decreased by advection effects by about a factor of several.