The Black Hole Accretion Code

TL;DR

The paper introduces BHAC, a new general-relativistic magnetohydrodynamics code for simulating black hole accretion, validated through tests and compared with existing codes, and coupled with a radiative transfer module for observational predictions.

Contribution

BHAC is a novel multidimensional code capable of simulating black hole accretion in arbitrary spacetimes with adaptive mesh refinement, enhancing accuracy and efficiency.

Findings

BHAC accurately reproduces known test problems.

Convergent accretion rates and fluxes in 3D simulations.

Synthetic images are consistent across resolutions.

Abstract

We present the black hole accretion code (BHAC), a new multidimensional general-relativistic magnetohydrodynamics module for the MPI-AMRVAC framework. BHAC has been designed to solve the equations of ideal general-relativistic magnetohydrodynamics in arbitrary spacetimes and exploits adaptive mesh refinement techniques with an efficient block-based approach. Several spacetimes have already been implemented and tested. We demonstrate the validity of BHAC by means of various one-, two-, and three-dimensional test problems, as well as through a close comparison with the HARM3D code in the case of a torus accreting onto a black hole. The convergence of a turbulent accretion scenario is investigated with several diagnostics and we find accretion rates and horizon-penetrating fluxes to be convergent to within a few percent when the problem is run in three dimensions. Our analysis also involves the study of the corresponding thermal synchrotron emission, which is performed by means of a new general-relativistic radiative transfer code, BHOSS. The resulting synthetic intensity maps of accretion onto black holes are found to be convergent with increasing resolution and are anticipated to play a crucial role in the interpretation of horizon-scale images resulting from upcoming radio observations of the source at the Galactic Center.