An accreting black hole model for sagittarius A*. II. a detailed study

Sgr A* is a unique, compact radio source at the Galactic center whose characteristics suggest that it may be a massive (i.e., ∼ 10⁶ M_⊙) black hole accreting from an ambient wind in that region. Earlier (simplified) calculations suggested that its 10⁸-10²⁰ Hz spectrum could be derived from bremsstrahlung and magnetic bremsstrahlung emission from plasma descending toward the event horizon at a rate of roughly 10²² g s^-1. Here, we introduce several significant improvements to the model, including (1) an exact treatment of the cyclotron/ synchrotron emissivity that is valid for all temperatures, (2) the actual determination of the temperature distribution in the inflow, and (3) the effect on the spectrum should the accreting plasma have a residual angular momentum, possibly forming a disk at small radii. We find that the most likely value of the mass in this improved model is ≈2 ± 1 × 10⁶ M_⊙, close to the range inferred earlier, but about a factor of 2 greater than the previous "best-fit" number. The main reason for this difference is that the more realistic (new) formulation of the magnetic bremsstrahlung emissivity has fluctuations with frequency that decrease the overall line-of-sight intensity, thereby pointing to a slightly larger mass in order to account for the observed spectrum. We also find that a slight excess of angular momentum in the accreting gas may be necessary in order to account for the IR luminosity from this source. Such an excess is consistent with the results of ongoing three-dimensional simulations that will be reported elsewhere.