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.