The X–ray Spectrum of the Dwarf Nova SS Cyg in quiescence and outburst

We reanalyse archival GINGA and ASCA X–ray data from SS Cyg in both quiescence and outburst, using multi–temperature plasma models for the continuum and line emission, together with their reflection from the X–ray illuminated white dwarf and accretion disk. Reflection is clearly detected in all the spectra, but its contribution is larger in the softer X–ray spectra seen in outburst than in quiescence. This supports models in which the quiescent inner disk is not present or not optically thick, so that the only reflector is the white dwarf surface rather than the white dwarf plus the disk. The amount of reflection in outburst is also more consistent with the hard X–rays forming a corona over the white dwarf surface rather than just an equatorial band. We detect partially ionized absorption in the ASCA outburst spectrum, which is probably the X–ray signature of the outflowing wind. The ASCA data also allow a detailed measure of the elemental abundances. We find that all detectable lines from the hot plasma (except perhaps Si and S) are a factor ∼ 2.5× weaker than expected from solar abundances. We examine possible deviations from coronal equilibrium, but conclude that the heavy elements are truly underabundant in SS Cyg. In quiescence the underlying intrinsic spectrum is consistent with a single temperature plasma. This conflicts with the expected cooling of the material, and requires that either the plasma is reheated or that the cooler emission is masked by absorption in an optically thin inner disk which affects only that part of the boundary layer emission below the disk. Such absorption is also required if the data are to match the theoretical models (Narayan & Popham 1993) for the quiescent hard X–ray emission. By contrast, the outburst spectrum is much softer and is dominated by the cooling components, so that it cannot be fit by a single temperature model. There are no readily available (or believable) theoretical models for the hard X–ray emission in outburst, but the emission observed can be described by material with a continuous temperature distribution. Although the outburst spectrum is much softer than the quiescent spectrum, the total luminosity of the X–ray component in both cases is similar, showing that the optically thin plasma emission is important even in outburst.