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 ∼3 × 106 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 ∼3r_s size of the emitter (where r_s ≡ 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)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.