A Simple Model for a Magnetic Reconnection–heated Corona

We construct a simple model for a magnetic reconnection heated corona above athin accretion disk in AGNs and Galactic black hole candidates(GBHCs). Thebasic assumptions are that (1) the magnetic reconnection heat is cooled downoverwhelmingly by Compton scattering in the corona and that (2) thermalconduction is dominantly cooled by evaporation of the chromospheric plasma inthe disk-corona interface before Compton cooling sets in. With these two basicequations as well as equi-partition of magnetic energy with gas energy in thedisk, we can consistently determine the fraction of accretion energy dissipatedin the corona without free parameters, and thus determine the temperature andall other quantities in both the corona and disk for given black hole mass andaccretion rate. Then, we calculate the luminosity contributed from the disk andcorona and the coronal flux weighted Compton $y$ parameter. It is found that,at a low luminosity (less than 0.1 Eddington luminosity) the spectrum is hardwith energy spectral index of around 1, while at a high luminosity (larger than0.1 Eddington luminosity) the spectrum can be either soft or hard. We also findthat the situation is almost same for supermassive and stellar-mass blackholes. These features are consistent with observations of AGNs and GBHCs.Comment: 8 pages including 2 figures. accepted by ApJ Lette