Self-Regulating Reconnection in Marginally Collisionless Coronae of Accreting Black Holes

Hard X-ray (up to $\sim$100 keV) emission is commonly observed in accreting Galactic (stellar-mass) and extra-galactic (super-massive) black hole sources. This emission is often attributed to the Comptonization of soft accretion-disk photons by a hot overlying corona with a Thomson optical depth is of order~1. We show that this observational result suggests that the coronal plasma is roughly marginally collisionless with respect to magnetic reconnection. As has been recently suggested for the Sun's corona,\footnote{D.~Uzdensky, ApJ, 671, 2139 (2007); see also Cassak~et~al., ApJ, 644, L145 (2006).} such marginal states may naturally result from a combination of disk-corona mass- and energy-exchange processes and the condition for the onset of fast collisionless reconnection. We also analyze the electron and ion cooling processes in a reconnection-heated corona, investigate the roles of pair creation and ion thermal conduction, and explore observational implications of our physical picture.