Liberation of specific angular momentum through radiation and scattering in relativistic black-hole accretion discs

TL;DR

This paper quantifies how electromagnetic radiation and scattering can remove angular momentum from relativistic black-hole accretion discs, highlighting their potential role in disc dynamics and galaxy feedback processes.

Contribution

It provides a relativistic calculation of angular momentum removal via radiation in accretion discs, extending the Novikov-Thorne model to include photon-driven angular momentum loss.

Findings

Radiation can remove >1% of angular momentum out to 10^4 Schwarzschild radii in maximally irradiated discs.

Thermal emission likely dominates angular momentum removal within ~100 Schwarzschild radii.

Effect of radiation is enhanced near spinning black hole horizons.

Abstract

A key component of explaining the array of galaxies observed in the Universe is the feedback of active galactic nuclei, each powered by a massive black hole's accretion disc. For accretion to occur, angular momentum must be lost by that which is accreted. Electromagnetic radiation must offer some respite in this regard, the contribution for which is quantified in this paper, using solely general relativity, under the thin-disc regime. Herein, I calculate extremised situations where photons are entirely responsible for energy removal in the disc and then extend and relate this to the standard relativistic accretion disc outlined by Novikov & Thorne, which includes internal angular-momentum transport. While there is potential for the contribution of angular-momentum removal from photons to be >~1% out to ~10^4 Schwarzschild radii if the disc is irradiated and maximally liberated of angular momentum through inverse Compton scattering, it is more likely of order 10^2 Schwarzschild radii if thermal emission from the disc itself is stronger. The effect of radiation/scattering is stronger near the horizons of fast-spinning black holes, but, ultimately, other mechanisms must drive angular-momentum liberation/transport in accretion discs.