General Relativistic Flux Modulations from Disk Instabilities in Sagittarius A*

Abstract

Near-IR and X-ray flares have been detected from the supermassive black hole Sgr A* at the center of our Galaxy with a (quasi-)period of ~17-20 minutes, suggesting an emission region only a few Schwarzschild radii above the event horizon. The latest X-ray flare, detected with XMM-Newton, is notable for its detailed light curve, yielding not only the highest quality period thus far, but also important structure reflecting the geometry of the emitting region. Recent MHD simulations of Sgr A*'s disk have demonstrated the growth of a Rossby wave instability that enhances the accretion rate for several hours, possibly accounting for the observed flares. In this Letter, we carry out ray-tracing calculations in a Schwarzschild metric to determine as accurately as possible the light curve produced by general relativistic effects during such a disruption. We find that the Rossby wave-induced spiral pattern in the disk is an excellent fit to the data, implying a disk inclination angle of ~77°. Note, however, that if this association is correct, the observed period is not due to the underlying Keplerian motion but rather to the pattern speed. The favorable comparison between the observed and simulated light curves provides important additional evidence that the flares are produced in Sgr A*'s inner disk.