General relativistic radiation hydrodynamics of accretion flows. I: Bondi-Hoyle accretion

TL;DR

This paper introduces a new general-relativistic radiation-hydrodynamics code to simulate accretion flows onto black holes, revealing significant differences from purely hydrodynamical models and providing self-consistent luminosity calculations.

Contribution

The study presents the first self-consistent simulation of Bondi-Hoyle accretion including radiation effects, improving understanding of accretion rates and luminosities in relativistic regimes.

Findings

Radiation pressure reduces accretion rates by about two orders of magnitude.

Luminosities remain super-Eddington despite reduced accretion rates.

Self-consistent luminosity estimates are significantly lower than traditional bremsstrahlung approximations.

Abstract

We present a new code for performing general-relativistic radiation-hydrodynamics simulations of accretion flows onto black holes. The radiation field is treated in the optically-thick approximation, with the opacity contributed by Thomson scattering and thermal bremsstrahlung. Our analysis is concentrated on a detailed numerical investigation of hot two-dimensional, Bondi-Hoyle accretion flows with various Mach numbers. We find significant differences with respect to purely hydrodynamical evolutions. In particular, once the system relaxes to a radiation-pressure dominated regime, the accretion rates become about two orders of magnitude smaller than in the purely hydrodynamical case, remaining however super-Eddington as are the luminosities. Furthermore, when increasing the Mach number of the inflowing gas, the accretion rates become smaller because of the smaller cross section of the black hole, but the luminosities increase as a result a stronger emission in the shocked regions. Overall, our approach provides the first self-consistent calculation of the Bondi-Hoyle luminosity, most of which is emitted within r~100 M from the black hole, with typical values L/L_Edd ~ 1-7, and corresponding energy efficiencies eta_BH ~ 0.09-0.5. The possibility of computing luminosities self-consistently has also allowed us to compare with the bremsstrahlung luminosity often used in modelling the electromagnetic counterparts to supermassive black-hole binaries, to find that in the optically-thick regime these more crude estimates are about 20 times larger than our radiation-hydrodynamics results.