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 spectra are never hard enough to match the observations. However, it has recently been pointed out that because of a thermal ionization instability, a hot "skin" forms on the top of the illuminated disk. Through numerical simulations in the slab corona geometry, we show that the completely ionized skin leads to a reduction in the reflected thermal blackbody component that amounts to a decrease in the Compton cooling rate of the corona and thus allows the X-ray spectra to be harder. While this brings the model closer to observations, in order for the predicted spectrum to be as hard as the observed spectra of Cyg X-1, the Thomson optical depth of the transition layer must be greater than 10, which is inconsistent with the ionization physics and observations. Therefore, the model with a planar corona covering the whole accretion disk is still strongly ruled out by the observations of GBHCs. Finally, we discuss accretion disks with magnetic flares (i.e., "patchy" corona) and show that the ionized skin resolves many of the arguments made in the literature against this model, although more quantitative future work is needed to test the model thoroughly.