Millimeter Flares and VLBI Visibilities from Relativistic Simulations of Magnetized Accretion onto the Galactic Center Black Hole

TL;DR

This study uses relativistic MHD simulations and radiative transfer to model the emission from Sgr A*, successfully matching VLBI observations and explaining millimeter flares through magnetic turbulence.

Contribution

First time time-dependent radiative transfer applied to relativistic MHD simulations of Sgr A* with direct comparison to VLBI data, constraining accretion rate and inclination.

Findings

Simulated visibilities match VLBI measurements when tuned for accretion rate.

Magnetic turbulence can produce millimeter flares with observed timescales and flux variations.

Constraints on accretion rate and inclination improve understanding of Sgr A*'s environment.

Abstract

The recent VLBI observation of the Galactic center black hole candidate Sgr A* at 1.3mm shows source structure on event-horizon scales. This detection enables a direct comparison of the emission region with models of the accretion flow onto the black hole. We present the first results from time-dependent radiative transfer of general relativistic MHD simulation data, and compare simulated synchrotron images at black hole spin a=0.9 with the VLBI measurements. After tuning the accretion rate to match the millimeter flux, we find excellent agreement between predicted and observed visibilities, even when viewed face-on (i < 30 degrees). VLBI measurements on 2000-3000km baselines should constrain the inclination. The data constrain the accretion rate to be (1.0-2.3)x10^-9 M_sun / yr with 99% confidence, consistent with but independent of prior estimates derived from spectroscopic and polarimetric measurements. Finally, we compute light curves, which show that magnetic turbulence can directly produce flaring events with .5 hour rise times, 2-3.5 hour durations and 40-50% flux modulation, in agreement with observations of Sgr A* at millimeter wavelengths.