X-ray flares from Sgr A*: star-disk interactions?

Abstract

Sgr A*, the putative black hole in our Galactic Center (GC), is extraordinary dim in all frequencies. Apparently the black hole is unable to accrete at the Bondi accretion rate for some reason. Another mystery of Sgr A* is the recently discovered large magnitude and short duration X-ray flares (Baganoff et al. 2001). Here we point out that X-ray flares should be expected from star passages through an inactive (i.e. formerly accreting) disk. There are thousands of stars in Sgr A* stellar cluster, and each star will pass through the disk twice per orbit. A shock hot enough to emit X-rays is formed around the star when the latter is inside the disk. We develop a simple yet somewhat detailed model for the X-ray emission from the shock. The duration of the flares, their X-ray spectra, frequency of events, and weakness of emission in the radio and near infra-red appear to be consistent with the available observations of X-ray flares. We therefore suggest that at least a fraction of the observed flares is due to the star-disk passages. Such star-disk flares are also of interest in the nuclei of nearby inactive galaxies, especially in connection with perspective Constellation-X and XEUS X-ray missions.