Soft X-ray pulsations from SS Cygni

Pulsed soft X-rays (0.1--0.5 keV) with a period of 9 s and a pulsed fraction that varies between 0 and 100% were detected from the dwarf nova SS Cygni at the peak of an optical outburst. This detection confirms for the first time the supposed high-energy origin of optical pulsations seen in erupting dwarf novae. The pulse shape is remarkably sinusoidal for such a large amplitude oscillation. The X-ray pulsation observed in this outburst is not coherent, in contrast to previous claims for the related optical oscillations. Instead, the phase of the pulsation apparently executes a random walk with a Q of approx.25, whereas the period is quite stable: Papprox.-1 x 10/sup -5/ s s/sup -1/. There is no evidence for any other periodic behavior on time scales between 160 ms and 3 hr. A blackbody fit to the observed spectrum yields Tapprox.3.5 x 10/sup 5/ K and a flux at the Earth of approx.4.5 x 10/sup -11/ ergs cm/sup -2/ s/sup -1/ in the 1/4 keV band. This in turn implies a total luminosity (integrated over all frequencies) at the source of approx.1.8 x 10/sup 33/ x (d/200 pc)/sup 2/ ergs s/sup -1/. Previous models for the optical oscillationsmore » fail to account for the properties of the X-ray pulsation. It is possible that an as yet unspecified instability in the boundary layer between the white dwarf and the acretion disk is the origin of the pulsation.« less