On the Nature of the Variable Infrared Emission from Sagittarius A*

Recent infrared (IR) observations of the center of our Galaxy indicate thatthe supermassive black hole source Sgr A* is strongly variable in the IR. Thetimescale for the variability, $\sim 30$ min, is comparable to that of theX-ray flares observed by {\em Chandra} and {\em XMM}, suggesting a commonphysical origin. In this paper, we investigate the nature of the IR emission inthe context of models recently proposed to interpret the X-ray flares. We showthat the IR emission in Sgr A* can be well explained by nonthermal synchrotronemission if a small fraction of the electrons in the innermost region of theaccretion flow around the black hole are accelerated {into a broken power-lawdistribution}, perhaps due to transient events such as magnetic reconnection.The model predicts differences in the variability amplitudes of flares in theIR and X-rays, in general agreement with observations. It also predicts thatthe IR emission should be linearly polarized, as has indeed been observedduring one epoch. IR and X-ray flares analogous to those observed in Sgr A* maybe detectable from other accreting SMBHs, provided $L \lsim 10^{-8} L_{EDD}$;at higher luminosities the flaring emission is dominated by thermalsynchrotron-self Compton emission, which is likely to be less variable.Comment: 17 pages, 5 figures, accepted by Ap