Non-thermal models for infrared flares from Sgr A∗

Abstract

Recent observations with mm very long baseline interferometry (mm-VLBI) and near-infrared (NIR) interferometry provide mm images and NIR centroid proper motion for Sgr A∗. Of particular interest are the NIR flares that have more than an order of magnitude higher flux density than the quiescent state. Here, we model the flares using time-dependent, axisymmetric, general relativistic magnetohydrodynamic (GRMHD) simulations with an electron distribution function that includes a small, variable, non-thermal component motivated by magnetic reconnection models. The models simultaneously match the observed mm mean flux density, mm image size, NIR quiescent flux density, NIR flare flux density, and NIR spectral slope. They also provide a better fit to the observed NIR flux density probability density function than previously reported models by reproducing the power-law tail at high flux density, though with some discrepancy at low flux density. Further, our modelled NIR image centroid shows very little movement: centroid excursions of more than 10 μas (the resolution of GRAVITY) are rare and uncorrelated with flux.