A ‘coronal-mass-ejection’ model for flares in Sagittarius A*

ABSTRACT High-resolution near-infrared observations with GRAVITY instrument have revealed rapid orbital motions of a hotspot around Sagittarius A* (Sgr A*), the supermassive black hole in our Galactic Centre, during its three bright flares. The projected distances of the spot to the black hole are measured and seem to increase with time. The values of distance, combined with the measured orbiting time, imply that the spot is rotating with a super-Keplerian velocity. These results are hard to understand if the spot stays within the accretion flow thus provides strong constraints on theoretical models for flares. Previously we have proposed a ‘CME’ model for the flares by analogy with the coronal-mass-ejection model in solar physics. In that model, magnetic reconnection occurred at the surface of the accretion flow results in the formation of flux ropes, which are then ejected out. Energetic electrons accelerated in the current sheet flow into the flux rope region and their radiation is responsible for the flares. In this paper, we apply the model to the interpretation of the GRAVITY results by calculating the dynamics of the ejected flux rope, the evolution of the magnetic field and the energy distribution of accelerated electrons, and the radiation of the system. We find that the model can well explain the observed light curve of the flares, the time-dependent distance, and the super-Keplerian motion of the hotspot. It also explains why the light curve of some flares has double peaks.