Numerical Simulations of Optically Thick Accretion onto a Black Hole - I. Spherical Case

TL;DR

This paper introduces a radiation transport module in a relativistic MHD code, testing it with shock tube and spherical accretion problems to study super-Eddington black hole accretion.

Contribution

The paper presents the implementation and validation of radiation transport in Cosmos++, enabling realistic simulations of radiative black hole accretion.

Findings

Radiation transport code shows good accuracy in shock tube tests.

Simulations of super-Eddington accretion yield luminosities below Eddington limit.

The code effectively models radiative processes in spherical accretion scenarios.

Abstract

Modeling the radiation generated by accreting matter is an important step towards realistic simulations of black hole accretion disks, especially at high accretion rates. To this end, we have recently added radiation transport to the existing general relativistic magnetohydrodynamic code, Cosmos++. However, before attempting to model radiative accretion disks, we have tested the new code using a series of shock tube and Bondi (spherical inflow) problems. The four radiative shock tube tests, first presented by Farris et al. (2008), have known analytic solutions, allowing us to calculate errors and convergence rates for our code. The Bondi problem only has an analytic solution when radiative processes are ignored, but is pertinent because it is closer to the physics we ultimately want to study. In our simulations, we include Thomson scattering and thermal bremsstrahlung in the opacity, focusing exclusively on the super-Eddington regime. Unlike accretion onto bodies with solid surfaces, super-Eddington accretion onto black holes does not produce super-Eddington luminosity. In our examples, despite accreting at up to 300 times the Eddington rate, our measured luminosity is always several orders of magnitude below Eddington.