A possible Rossby wave instability origin for the flares in Sagittarius A

In recent years, near-IR and X-ray flares have been detected from the Galaxy's central radio point source, Sagittarius A* (Sgr A*), believed to be an similar to 3 x 10(6) M-circle dot supermassive black hole. In some cases, the transient emission appears to be modulated with a (quasi-) periodic oscillation (QPO) of similar to 17-20 minutes. The implied similar to 3r(s) size of the emitter (where r(s) equivalent to 2GM/c(2) is the Schwarzschild radius) points to an instability-possibly induced by accretion - near the marginally stable orbit (MSO) of a slowly spinning object. But Sgr A* is not accreting via a large, standard disk; instead, the low-density environment surrounding it apparently feeds the black hole with low angular momentum clumps of plasma that circularize within similar to(10-300)r(s) and merge onto a compact, hot disk. In this Letter, we follow the evolution of the disk following such an event, and we show that a Rossby wave instability, particularly in its magnetohydrodynamic (MHD) form, grows rapidly and produces a period of enhanced accretion lasting several hours. Both the amplitude of this response and its duration match the observed flare characteristics rather well.