X-ray emission from non-magnetic cataclysmic variables

Abstract

Many (∼50) cataclysmic variables, binary systems in which a white dwarf accretes from a close companion star, are now known to be hard (≳2 keV) and variable X-ray sources1. While some of these ∼15) exhibit regular pulsing in X rays, suggesting that they have magnetic fields strong enough to channel the accretion flow onto restricted parts of the white dwarf surface (the AM Her and ‘intermediate polar’ systems), the majority do not, and hence presumably have rather weaker fields (B≲105 G). SS Cygni, the brightest object of this ‘non-magnetic’ class, exhibits a combination of systematic features in its X-ray and optical emission which existing models are only partially successful in reproducing. We suggest here that at sufficiently low accretion rates a thermal instability in the accretion flow leads to the formation of a hard X-ray-emitting corona around the white dwarf; the corona is relatively suppressed at higher accretion rates. We show that the cooling of such a corona gives the observed correlation between X-ray temperature and luminosity, and that hard X-ray production may be rather inefficient because of transport processes. These can give rise to soft X-ray emission from most of the white dwarf surface, with a correspondingly low effective temperature.