A Simple Model for a Magnetic Reconnection–heated Corona

Abstract

We construct a simple model for a magnetic reconnection-heated corona above a thin accretion disk in active galactic nuclei (AGNs) and Galactic black hole candidates (GBHCs). The basic assumptions are that (1) the magnetic reconnection heat is cooled down overwhelmingly by Compton scattering in the corona and (2) the thermal conduction is dominantly cooled by evaporation of the chromospheric plasma in the disk-corona interface before Compton cooling sets in. With these two basic equations as well as equipartition of magnetic energy with gas energy in the disk, we can consistently determine the fraction of accretion energy dissipated in the corona without free parameters and thus determine the temperature and all other quantities in both the corona and disk for a given black hole mass and accretion rate. Then, we calculate the luminosity contributed from the disk and corona and the coronal flux-weighted Compton y-parameter. It is found that at low luminosity (L < 0.1LEdd) the spectrum is hard, with a spectral index of ?sp ~ 1 (f? ?), while at high luminosity (L 0.1LEdd) the spectrum can be either soft or hard. We also find that the situation is almost the same for supermassive and stellar mass black holes. These features are consistent with observations of AGNs and GBHCs.