Line Emission from Cooling Accretion Flows in the Nucleus of M 31

The recent Chandra X-ray observations of the nucleus of M31, combined with earlier VLA radio and HST UV spectral measurements, provide the strictest constraints thus far available on the nature of accretion onto the supermassive black hole (called M31* hereafter) in this region. One of the two newly-detected sources within roughly an arcsec of M31* may be its X-ray counterpart. If not, the X-ray flux from the nucleus must be even lower than inferred previously. Some uncertainty remains regarding the origin of the UV excess from the compact component known as P2. In our earlier analysis, we developed a unified picture for the broadband spectrum of this source and concluded that M31* could be understood on the basis of the accretion model for Sgr A* at the Galactic center, though with several crucial differences. Contrary to the ‘standard’ picture in which the infalling plasma attains temperatures in excess of 10 K near the event horizon, the best fit model for M31*, under the assumption that the UV radiation is in fact produced by this source, appears to correspond to a cool branch solution, arising from strong line cooling inside the capture radius. Starting its infall with a temperature of about 10 K in the post-shock region, the plasma cools down efficiently to about 10 K toward smaller radii. An important prediction of this model is the appearance of a prominent UV spike from hydrogen line emission, which for simplicity was handled only crudely in the earlier work. It is our purpose here to model this line emission with significantly greater accuracy, using the algorithm CLOUDY, and to correctly take into account the attenuation along the line of sight. We show that this level of sophistication reproduces the currently available multi-wavelength observations very well. Very importantly, we predict a spectrum with several additional prominent emission lines that can be used to verify the model with future high-resolution observations. A non-detection of the predicted line emission from M31* would then tilt the favored accretion picture in the direction of a hot Sgr A*-type of model, though with only a single point remaining in the spectrum of M31*,