Sgr A* X-ray flares from non-thermal particle acceleration in a magnetically arrested disc

TL;DR

This paper models Sgr A*'s X-ray and infrared flares as resulting from non-thermal particle acceleration in a magnetically arrested disc, using PIC and GRMHD simulations to reproduce observed flare properties.

Contribution

It combines PIC and GRMHD simulations to connect magnetic reconnection in MADs with observable X-ray and infrared flares from Sgr A* in a novel way.

Findings

MAD flux eruptions can accelerate particles to high energies

Model reproduces quiescent and flaring X-ray luminosities

Infrared flares mainly due to temperature increases near the black hole

Abstract

Sgr A* exhibits flares in the near-infrared and X-ray bands, with the luminosity in these bands increasing by factors of $10-100$ for $\approx 60$ minutes. One of the models proposed to explain these flares is synchrotron emission of non-thermal particles accelerated by magnetic reconnection events in the accretion flow. We use the results from PIC simulations of magnetic reconnection to post-process 3D two-temperature GRMHD simulations of a magnetically arrested disc (MAD). We identify current sheets, retrieve their properties, estimate their potential to accelerate non-thermal particles and compute the expected non-thermal synchrotron emission. We find that the flux eruptions of MADs can provide suitable conditions for accelerating non-thermal particles to energies $\gamma_e \lesssim 10^6$ and producing simultaneous X-ray and near-infrared flares. For a suitable choice of current-sheet parameters and a simpified synchrotron cooling prescription, the model can simultaneously reproduce the quiescent and flaring X-ray luminosities as well as the X-ray spectral shape. While the near-infrared flares are mainly due to an increase in the temperature near the black hole during the MAD flux eruptions, the X-ray emission comes from narrow current sheets bordering highly magnetized, low-density regions near the black hole and equatorial current sheets where the flux on the black hole reconnects. As a result, not all infrared flares are accompanied by X-ray ones. The non-thermal flaring emission can extend to very hard ($\lesssim 100$ keV) X-ray energies.