A Model for the Galactic Population of Binary Supersoft X-Ray Sources

Abstract

A population synthesis model for the Galactic binary supersoft X-ray sources with white dwarf accretors is derived. It is shown that three major subpopulations may exist: with low-mass main-sequence donors (˜500-850 objects), with low-mass subgiant donors (˜460 objects), and with (super)giant donors (˜500-600 objects). The orbital period ranges of the three groups of sources are 80 minutes-12 hr, 10 hr-20 days, and 10 days-10 yr, respectively. Each of these families contains both permanent and recurrent sources, but in different proportions. The former are steady hydrogen-burning white dwarfs, while the latter are white dwarfs in a post-nova explosion state, burning the remainders of hydrogen after a thermonuclear runaway. The estimated number of recurrent sources in the "on" state is a sensitive function of the assumptions on the duration of the supersoft emission stage. Less populated families include planetary nebula nuclei, double degenerate systems and "helium Algols." For all the major groups of sources, we derive distributions over potentially observable parameters: orbital periods, masses of components, and "on" times. Intrinsic absorption of supersoft X-rays by circumbinary matter and the low transparency of the interstellar medium reduce the number of "detectable" galactic sources to ˜20-30, a considerable proportion of which have probably already been detected. It is estimated that the total rate at which accreting white dwarfs in Galactic binaries reach the Chandrasekhar mass is ˜3 > 10-5 yr-1. The only type of systems in which the accumulation of a He shell which is able to detonate is possible, are the systems with subgiant donors, in which such events occur at the rate of ˜3 x 10-4 yr-1.