Attenuation of supersoft X-ray sources by circumstellar material

Recent studies have suggested the possibility of significantly obscuring supersoft X-ray sources in relatively modest amounts of local matter lost from the binaries themselves. If correct, then this would have explained the paucity of observed supersoft X-ray sources and would have significance for the search for single-degenerate Type Ia supernova progenitors. We point out that earlier studies of circumbinary obscuration ignored photoionizations of the gas by the emission from the supersoft X-ray source. We revisit the problem using a full, self-consistent calculation of the ionization state of the circumbinary material photoionized by the radiation of the central source. Our results show that the circumstellar mass-loss rates required for obscuration of supersoft X-ray sources is about an order of magnitude larger than those reported in earlier studies, for comparable model parameters. While this does not entirely rule out the possibility of circumstellar material obscuring supersoft X-ray sources, it makes it unlikely that this effect alone can account for the majority of the missing supersoft X-ray sources. We discuss the observational appearance of hypothetical obscured nuclear-burning white dwarfs and show that they have signatures making them distinct from photoionized nebulae around supersoft X-ray sources imbedded in the low-density interstellar medium.