SIMBAD references

The compact, nonthermal radio source Sgr A* is believed to be a supermassive blackhole at the Galactic Center. Spectral and kinematic studies give confirming indications that its mass may be close to 10⁶ M_Sun. Most of the radio and high-energy emission from this source appears to originate from within a quasi-spherical infall (extending out to ∼3x10¹⁶cm), whereas the recently-detected IR luminosity is probably due to a small accretion disk (formed within 50-100 Schwarzschild radii of the compact object), indicating that the inflowing plasma may retain some angular momentum, forcing it to circularize before reaching the event horizon. We report the results of the first 3D Bondi-Hoyle numerical simulations of this accretion process, focusing on the hydrodynamics of the inflow. For a uniform medium, we find that most of the specific angular momentum l is cancelled in the post-bow-shock region, though local fluctuations in the accretion rate (as large as 10-20% on a time scale of less than 10 years), produce transient excesses in l that lead to alternately prograde and retrograde disk structures. This variability suggests that the radio, K and H band fluxes from Sgr A* should show significant fluctuations over a period of several years or less. Observations over the past decade indicate that this is apparently the case in the radio. Future IR studies may confirm this at shorter wavelengths as well. Our calculations suggest that because of the bow-shock structure and the changing physical conditions with radius, the source shape should have a frequency-dependent axial-ratio, which should increase with increasing wavelength. This appears to be consistent with recent radio observations at 1 and 20 cm.