On double-degenerate type Ia supernova progenitors as supersoft X-ray sources

A population synthesis analysis using SeBa

¹ Department of Astrophysics/IMAPPRadboud University Nijmegen, PO box 9010, 6500 GL Nijmegen, The Netherlands
² Institute for Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
³ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str.1, Postfach 1317, 85741 Garching, Germany
e-mail: mede@mpa-garching.mpg.de

Received: 17 April 2013
Accepted: 10 October 2013

Abstract

Context. The nature of the progenitors of type Ia supernova progenitors remains unclear. While it is usually agreed that single-degenerate progenitor systems would be luminous supersoft X-ray sources, it was recently suggested that double-degenerate progenitors might also go through a supersoft X-ray phase.

Aims. We aim to examine the possibility of double-degenerate progenitor systems being supersoft X-ray systems, and place stringent upper limits on the maximally possible durations of any supersoft X-ray source phases and expected number of these systems in a galactic population.

Methods. We employ the binary population synthesis code SeBa to examine the mass-transfer characteristics of a possible supersoft X-ray phase of double-degenerate type Ia supernova progenitor systems for 1) the standard SeBa assumptions; and 2) an optimistic best-case scenario. The latter case establishes firm upper limits on the possible population of supersoft source double-degenerate type Ia supernova progenitor systems.

Results. Our results indicate that unlike what is expected for single-degenerate progenitor systems, the vast majority of the material accreted by either pure wind mass transfer or a combination of wind and RLOF mass transfer is helium rather than hydrogen. Even with extremely optimistic assumptions concerning the mass-transfer and retention efficiencies, the average mass accreted by systems that eventually become double-degenerate type Ia supernovae is small. Consequently, the lengths of time that these systems may be supersoft X-ray sources are short, even under optimal conditions, and the expected number of such systems in a galactic population is negligible.

Conclusions. The population of double-degenerate type Ia supernova progenitors that are supersoft X-ray sources is at least an order of magnitude smaller than the population of single-degenerate progenitors expected to be supersoft X-ray sources, and the supersoft X-ray behaviour of double-degenerate systems typically ceases long before the supernova explosions.