Quasi-Coherent Oscillations in the Extreme Ultraviolet Flux of the Dwarf Nova SS Cygni
Quasi-coherent oscillations have been detected in the extreme ultraviolet flux of the dwarf nova SS Cygni during observations with the Extreme Ultraviolet Explorer satellite of the rise and plateau phases of an anomalous outburst in 1993 August and a normal outburst in 1994 June/July. On both occasions, the oscillation turned on during the rise to outburst and persisted throughout the observation. During the 1993 outburst, the period of the oscillation fell from 9.3 s to 7.5 s over an interval of 4.4 days; during the 1994 outburst, the period fell from 8.9 s to 7.19 s (the shortest period ever observed in SS Cyg, or any other dwarf nova) within less than a day, and then rose to 8.0 s over an interval of 8.0 days. For both outbursts, the period $P$ of the oscillation was observed to correlate with the 75--120 Å\ count rate $I_{\rm EUV}$ according to $P\propto I_{\rm EUV}^{-0.094}$. A magnetospheric model is considered to reproduce this variation. It is found that an effective high-order multipole field is required, and that the field strength at the surface of the white dwarf is 0.1--1 MG. Such a field strength is at the lower extreme of those measured or inferred for bona fide magnetic cataclysmic variables.