The EXOSAT observations of SS Cygni

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Abstract
A series of nine observations of SS Cygni made using EXOSAT between 1983 November and 1984 September are presented. Distinct soft (≪ 1 keV) and hard (> 1 keV) X-ray emitting components were spectrally resolved by the LE and ME instruments, respectively. In quiescence the source shows hard X-ray emission with a 2–10 keV luminosity of 1.2 × 1033 (d/200 pc)2 erg s−1. The hard X-ray emission is suppressed in outburst but shows no significant increase in photoelectric absorption (⁠|$N_\text H \lt3\times10^{21}\enspace\text {cm}^{-2}$|⁠). The soft X-ray emission approximately follows the optical behavior, but the outburst is delayed by 0.5−1.1 d from the optical. The rise and decay times of outburst are shorter in the soft X-ray band with respect to the optical. During outburst, the soft X-ray component is strongly modulated at a period which varies (in these observations) between 10.4 and 7.4 s. At the onset of soft X-ray outburst the oscillator is observed to spin-up with |$\dot P = - 8 \times 10^{-5}$|⁠. At this stage there is a strong correlation between period and luminosity. Near the peak of outburst the oscillator is observed to be spinning down at a rate of |$\dot P = - 4.6 \times 10^{-6}$|⁠, which increases to 1.3 × 10−5 as the X-ray outburst returns to quiescence. The behaviour of the oscillator has been studied in an analogous fashion to previous investigations. In spindown the phase is continuous with time, but the period seems to jump by discrete amounts (up to ∼0.07 s) on time-scales of a few × 1000 s. We find that a mathematical description of the oscillator given by Córdova et al. is a poor representation of the observed behaviour.

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