An accretion-induced X-ray flare in Sagittarius A*

The recent detection of a 3 hr X-ray flare from Sgr A* by Chandra provides very strong evidence for a compact emitting region near this supermassive black hole at the Galactic center. The millimeter/submillimeter spectrum and linear polarimetric properties of Sgr A* and its quiescent state X-ray flux density are consistent with a model in which low angular momentum gas captured at large radii circularizes to form a hot magnetized Keplerian flow within tens of Schwarzschild radii of the black hole's event horizon. In the quiescent state of Sgr A*, the X-ray emission appears to be produced by self-Comptonization of the millimeter/submillimeter synchrotron photons emitted in this region. In this Letter we show that the prominent X-ray flare seen in Sgr A* may be due to a sudden enhancement of accretion through the circularized flow. Depending on whether the associated response of the anomalous viscosity is to increase or decrease in tandem with this additional injection of mass, the X-ray photons during the outburst may be produced via either thermal bremsstrahlung (if the viscosity decreases) or synchrotron self-Comptonization (SSC; if the viscosity increases). However, the latter predicts a softer X-ray spectrum than was seen by Chandra, so it appears that a bremsstrahlung origin for the X-ray outburst is favored. A strong correlation is expected between the millimeter/submillimeter and X-ray fluxes when the flare X-rays are produced by SSC, while the correlated variability is strongest between the submillimeter/far-IR and X-rays when bremsstrahlung emission is dominant during the flare. In addition, we show that future coordinated multiwavelength observations planned for the 2002 and 2003 cycles may be able to distinguish between the accretion and jet scenarios.