Global GRMHD Simulations of Black Hole Accretion Flows: a Convergence Study

TL;DR

This study evaluates the convergence of global GRMHD simulations of black hole accretion disks across different resolutions, focusing on key diagnostics and synthetic spectra to determine the reliability of simulation results.

Contribution

It provides a detailed convergence analysis of GRMHD models with varying resolutions, identifying the resolution needed for reliable simulation diagnostics and spectra.

Findings

Shell-averaged temperature nearly independent of resolution

Plasma beta shows signs of convergence with increased resolution

Synthetic spectra are consistent across resolutions except at lowest

Abstract

Global, general relativistic magnetohydrodynamic (GRMHD) simulations of nonradiative, magnetized disks are widely used to model accreting black holes. We have performed a convergence study of GRMHD models computed with HARM3D. The models span a factor of 4 in linear resolution, from 96x96x64 to 384x384x256. We consider three diagnostics of convergence: (1) dimensionless shell-averaged quantities such as plasma \beta; (2) the azimuthal correlation length of fluid variables; and (3) synthetic spectra of the source including synchrotron emission, absorption, and Compton scattering. Shell-averaged temperature is, except for the lowest resolution run, nearly independent of resolution; shell-averaged plasma \beta\ decreases steadily with resolution but shows signs of convergence. The azimuthal correlation lengths of density, internal energy, and temperature decrease steadily with resolution but show signs of convergence. In contrast, the azimuthal correlation length of magnetic field decreases nearly linearly with grid size. We argue by analogy with local models, however, that convergence should be achieved with another factor of 2 in resolution. Synthetic spectra are, except for the lowest resolution run, nearly independent of resolution. The convergence behavior is consistent with that of higher physical resolution local model (shearing box) calculations and with the recent nonrelativistic global convergence studies of Hawley et al. (2011).