The EUV/X-ray spectrum of SS Cygni in outburst

Abstract

The dwarf nova SS Cyg was observed during decline from outburst by the ROSAT observatory during its all-sky survey. In addition to data from the ROSAT Wide Field Camera (WFC) and Position Sensitive Proportional Counter (PSPC), we acquired two blocks of hard X-ray data from the Ginga Large Area Counter, extending our EUV/X-ray coverage over the band 0.06-15 keV. This wide band allows us to discriminate clearly the presence of two components in the spectrum: a soft component which decays more quickly than the optical flux, and a hard X-ray component which shows no overall trend through our observations. The soft component dominates the spectrum below 0.5 keV during outburst, and can be modelled as a blackbody with a temperature T=20-25 eV. Its luminosity appears to be much less than that of the accretion disc, but considerably greater than the hard component luminosity, which is suppressed by a factor of four compared with that seen in quiescence. A strong iron emission line and absorption edge in the Ginga spectrum imply substantial covering of the hard X-ray emission region by highly ionized gas, which is hard to reconcile with the standard picture in which the hard X-rays arise from a hot corona.