HYDRODYNAMICAL 3D BONDI-HOYLE ACCRETION ONTO THE GALACTIC-CENTER BLACKHOLE CANDIDATE SGR-A

The compact, nonthermal radio source Sgr A is believed to be a superm assive blackhole at the Galactic Center. Spectral and kinematic studie s give confirming indications that its mass may be close to 10(6) M.. Most of the radio and high-energy emission from this source appears to originate from within a quasi-spherical infall (extending out to appr oximately 3 x 10(16) cm), whereas the recently-detected IR luminosity is probably due to a small accretion disk (formed within 50-100 Schwar zschild radii of the compact object), indicating that the inflowing pl asma may retain some angular momentum, forcing it to circularize befor e reaching the event horizon. We report the results of the first 3D Bo ndi-Hoyle numerical simulations of this accretion process, focusing on the hydrodynamics of the inflow. For a uniform medium, we find that m ost of the specific angular momentum l is cancelled in the post-bow-sh ock region, though local fluctuations in the accretion rate (as large as 10-20% on a time scale of less than 10 years), produce transient ex cesses in l that lead to alternately prograde and retrograde disk stru ctures. This variability suggests that the radio, K and H band fluxes from Sgr A should show significant fluctuations over a period of seve ral years or less. Observations over the past decade indicate that thi s is apparently the case in the radio. Future IR studies may confirm t his at shorter wavelengths as well. Our calculations suggest that beca use of the bow-shock structure and the changing physical conditions wi th radius, the source shape should have a frequency-dependent axial-ra tio, which should increase with increasing wavelength. This appears to be consistent with recent radio observations at 1 and 20 cm.