Obscuration of supersoft X-ray sources by circumbinary material

Abstract

The progenitors of supernovae (SNe) type Ia are usually assumed to be either a single white dwarf (WD) accreting from a non-degenerate companion (the SD channel) or the result of two merging WDs (DD channel). However, no consensus currently exists as to which progenitor scenario is the correct one, or whether the observed SN Ia rate is produced by a combination of both channels. Unlike a DD progenitor a SD progenitor is expected to emit supersoft X-rays for a prolonged period of time (~1 Myr) as a result of the burning of accreted matter on the surface of the WD. An argument against the SD channel as a significant producer of SNe type Ia has been the lack of observed supersoft X-ray sources (SSS) and the lower-than-expected integrated soft X-ray flux from elliptical galaxies. We wish to determine if it is possible to obscure the supersoft X-ray emission from a nuclear burning white dwarf in an accreting single degenerate binary system. In case of obscured systems we wish to determine their general observational characteristics. We examine the emergent X-ray emission from a canonical SSS system surrounded by a spherically symmetric configuration of material, assuming a black body spectrum with T_BB=50 eV and L=10^38 erg/s. The circumbinary material is assumed to be of solar chemical abundances, and we leave the mechanism behind the mass loss into the circumbinary region unspecified. If steadily accreting, nuclear burning WDs are canonical SSS our analysis suggests that they can be obscured by relatively modest circumbinary mass loss rates. This may explain the discrepancy of SSS compared to the SN Ia rate inferred from observations if the SD progenitor scenario contributes significantly to the SN Ia rate. Recycled emissions from obscured systems may be visible in other wavebands than X-rays. It may also explain the lack of observed SSS in symbiotic binary systems.