Revisiting flares in Sagittarius A* based on general relativistic magnetohydrodynamic numerical simulations of black hole accretion

TL;DR

This paper uses general relativistic magnetohydrodynamic simulations to analyze the flares in Sagittarius A*, modeling electron acceleration and radiation to match observed light curves and polarization.

Contribution

It introduces a simulation-based model of Sgr A* flares that directly interprets observational data through flux rope identification and electron radiation calculations.

Findings

Simulation results match observed flare light curves.

Polarization patterns are consistent with observations.

Flux rope formation due to magnetic reconnection explains flare activity.

Abstract

High-resolution observations with GRAVITY-VLTI instrument have provided abundant information about the flares in Sgr A*, the supermassive black hole in our Galactic center, including the time-dependent location of the centroid (a "hot spot"), the light curve, and polarization. Yuan et al. (2009) proposed a "coronal mass ejection" model to explain the flares and their association with the plasma ejection. The key idea is that magnetic reconnection in the accretion flow produces the flares and results in the formation and ejection of flux ropes. The dynamical process proposed in the model has been confirmed by three-dimensional GRMHD simulations in a later work. Based on this scenario, in our previous works the radiation of the flux rope has been calculated analytically and compared to the observations. In the present paper, we develop the model by directly using numerical simulation data to interpret observations. We first identify flux ropes formed due to reconnection from the data. By assuming that electrons are accelerated in the reconnection current sheet and flow into the flux rope and emit their radiation there, we have calculated the time-dependent energy distribution of electrons after phenomenologically considering their injection due to reconnection acceleration, radiative and adiabatic cooling. The radiation of these electrons is calculated using the ray-tracing approach. The trajectory of the hot spot, the radiation light curve during the flare, and the polarization are calculated. These results are compared with the GRAVITY observations and good consistencies are found.