ar X iv : a st ro - p h / 05 11 52 0 v 1 1 7 N ov 2 00 5 A Possible Rossby Wave Instability Origin for the Flares in

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 a ∼ 3 × 10 M⊙ supermassive black hole. In some cases, the transient emission appears to be modulated with a (quasi-)periodic oscillation (QPO) of ∼ 17 − 20 minutes. The implied ∼ 3 rS size of the emitter (where rS ≡ 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 ∼ 10− 300 rS and merge onto a compact, hot disk. In this Letter, we follow the evolution of the disk following such an event, and 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. Subject headings: accretion—black hole physics—Galaxy: center—magnetohydrodynamics—plasmas—