Angular momentum transport by thermal emission in black hole accretion disks

TL;DR

This paper investigates how thermal photons emitted from a black hole accretion disk carry away angular momentum, contributing to accretion dynamics alongside viscosity, with a specific quantification of this effect.

Contribution

It introduces a calculation of angular momentum loss via thermal emission in black hole disks, highlighting its role in accretion processes.

Findings

Thermal emission causes a significant angular momentum loss (~0.4r_s/r) at radius r.

The effect is similar to the Poynting-Robertson effect.

Thermal emission contributes to accretion but cannot drive it alone.

Abstract

We calculate the amount of angular momentum that thermal photons carry out of a viscous black hole accretion disc, due to the strong Doppler shift imparted to them by the high orbital velocity of the radiating disc material. While the emission of radiation can not drive accretion on its own, we find that it does result in a loss of specific angular momentum, thereby contributing to an otherwise viscosity-driven accretion flow. In particular, we show that the fraction of the angular momentum that is lost to thermal emission at a radius r in a standard, multi-color disc is ~ 0.4r_s/r, where r_s is the Schwarzschild radius of the black hole. We briefly highlight the key similarities between this effect and the closely related Poynting-Robertson effect.