The role of an extended corona in the formation of emission lines and continuum in active galactic nuclei

We study a model of an accretion disc surrounded by an extended corona with a temperature of 10(7)-10(8) K. This corona modifies the disc spectrum since it redirects a significant fraction of the emission from the central parts towards the more distant parts of the disc. The same corona is indirectly the source of the broad emission lines because we expect the formation of cool clouds at the base of the corona as a result of thermal instabilities. We model the number of clouds and their motion through the corona, adopting a few different physically sound assumptions. Comparing the predicted optical/UV continua and emission-line ratios and profiles with the observed values, we favour a particular model of a typical quasar. It radiates at similar to 0.5 of the Eddington luminosity, the corona surrounding the disc is heated in excess of the inverse Compton heating, and the broad-line clouds are most probably destroyed when accelerated vertically above the speed of sound within the corona.