Binary population synthesis study on supersoft X-ray phase of single   degenerate type Ia supernova progenitors
  In the single degenerate (SD) scenario for type Ia supernovae (SNe Ia), a mass-accreting white dwarf is expected to experience a supersoft X-ray source (SSS) phase. However, some recent observations showed that the expected number of the mass-accreting WD is much lower than that predicted from theory, whatever in spiral or elliptical galaxies. In this paper, we did a binary population synthesis study on the relative duration of the SSS phase to their whole mass-increasing phase of WDs leading to SNe Ia. We found that for about 40% progenitor systems, the relative duration is shorter than 2% and the evolution of the mean relative duration shows that it is always smaller than 5%, whatever for young or old SNe Ia. In addition, before SNe Ia explosion, more than 55% progenitor systems are experiencing a dwarf novae phase, and only no more than 10% is staying SSS phase. These results are consistent with the recent observations, and imply that both in early- and late-type galaxies, only a small fraction of mass-accreting WD resulting in SNe Ia contribute to the supersoft X-ray flux. So, although our results are not directly related to the X-ray output of SN Ia progenitor, the low supersoft X-ray luminosity observed in early type galaxies may have no ability to exclude the validity of SD model. On the contrary, it is evidence to support the SD scenario. 
Xiangcun Meng, Wuming Yang
astro-ph.SR