Correlation between Flux and Spectral Index during Flares in Sagittarius A*

Flares in Sagittarius A* are produced by hot plasmas within a fewSchwarzschild radii of the supermassive black hole at the Galactic center. Therecent detection of a correlation between the spectral index and flux during anear infrared (NIR) flare provides a means to conduct detailed investigationsof the plasma heating and radiation processes. We study the evolution of theelectron distribution function under the influence of a turbulent magneticfield in a hot collisionless plasma. The magnetic field, presumably generatedthrough instabilities in the accretion flow, can both heat the plasma viaresonant wave-particle coupling and cool the electrons via radiation. Theelectron distribution can generally be approximated as relativistic Maxwellian.To account for the observed correlation, we find that the magnetic field needsto be anti-correlated with the electron ''temperature''. NIR and X-ray lightcurves are produced for a cooling and a heating phase. The model predictssimultaneous flare activity in the NIR and X-ray bands, which can be comparedwith observations. These results can be applied to MHD simulations to study theradiative characteristics of collisionless plasmas, especially accretion flowsin low-luminosity AGNs.Comment: 26 pages, 11 figure