Obscuration of supersoft X-ray sources by circumbinary material

A way to hide Type Ia supernova progenitors?

¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO box 9010, 6500 GL Nijmegen, The Netherlands
e-mail: m.nielsen@astro.ru.nl
² Sterrenkundig Instituut Anton Pannekoek, Universiteit van Amsterdam (UvA), PO box 94249, 1090 GE Amsterdam, The Netherlands
³ Institute for Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 8 March 2012
Accepted: 23 July 2012

Abstract

Context. The progenitors of Type Ia supernovae are usually assumed to be either a single white dwarf accreting from a non-degenerate companion (the single-degenerate channel) or the result of two merging white dwarfs (the double degenerate channel). However, no consensus currently exists as to which progenitor scenario is the correct one, or whether the observed Type Ia supernovae rate is produced by a combination of both channels. Unlike a double degenerate progenitor, a single-degenerate 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 white dwarf. An argument against the single-degenerate channel as a significant producer of Type Ia supernovae has been the lack of observed supersoft X-ray sources and the lower-than-expected integrated soft X-ray flux from elliptical galaxies.

Aims. We wish to determine whether it is possible to obscure the supersoft X-ray emission from a nuclear-burning white dwarf in an accreting single-degenerate binary system. In the case of obscured systems we wish to determine their general observational characteristics.

Methods. We examine the emergent X-ray emission from a canonical supersoft X-ray system surrounded by a spherically symmetric configuration of material, assuming a black-body spectrum with T_bb = 50 eV and L = 10³⁸ erg s^-1. 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.

Results. We find that relatively low circumstellar mass-loss rates, Ṁ = 10^-9−10^-8 M_⊙ yr^-1, at binary separations of  ~1 AU or less, will cause significant attenuation of the X-rays from the supersoft X-ray source. These circumstellar mass-loss rates are sufficient to make a canonical supersoft X-ray source in typical external galaxies unobservable in Chandra.

Conclusions. If steadily accreting, nuclear-burning white dwarfs are canonical supersoft X-ray sources our analysis suggests that they can be obscured by relatively modest circumbinary mass-loss rates. This may explain the discrepancy of supersoft sources relative to the Type Ia supernova rate inferred from observations if the single-degenerate progenitor scenario contributes significantly to the Type Ia supernova rate. Recycled emissions from obscured systems may be visible in wavebands other than X-rays. It may also explain the lack of observed supersoft sources in symbiotic binary systems.