Accretion disks with coronae in Cygnus X-1: The role of a transition layer

Several recent papers have shown that an accretion disk corona model, where a slab corona sandwiches the cold accretion disk, is problematic for hard- state spectra of galactic black hole candidates (GBHCs) because the model s pectra are never hard enough to match the observations. However, it has rec ently been pointed out that because of a thermal ionization instability, a hot "skin" forms on the top of the illuminated disk. Through numerical simu lations in the slab corona geometry, we show that the completely ionized sk in leads to a reduction in the reflected thermal blackbody component that a mounts to a decrease in the Compton cooling rate of the corona and thus all ows the X-ray spectra to be harder. While this brings the model closer to o bservations, in order for the predicted spectrum to be as hard as the obser ved spectra of Cyg X-1, the Thomson optical depth of the transition layer m ust be greater than 10, which is inconsistent with the ionization physics a nd observations. Therefore, the model with a planar corona covering the who le accretion disk is still strongly ruled out by the observations of GBHCs. Finally, we discuss accretion disks with magnetic flares (i.e., "patchy" c orona) and show that the ionized skin resolves many of the arguments made i n the literature against this model, although more quantitative future work is needed to test the model thoroughly.