The soft X-ray superoutburst of VW Hydri.

Abstract

The dwarf nova VW Hyi has been monitored with Exosat, during and shortly after the optical superoutburst of November 1983. VW Hyi appears to be a very soft X-ray emitter, and is marginally detected in the hard (1 to 6 keV) X-ray range. The soft X-ray emission increased by two orders of magnitude during outburst, with the dominant emission at wavelengths longer than 170 A. VW Hyi is now the third dwarf-nova system, in which a strong soft X-ray component is detected during outburst. An increase of the soft X-ray flux was observed, with a delay of 2.5 days with respect to the onset of the optical outburst. This delay is larger than the typical delay of about 1 day between the UV/soft X-ray outburst and optical outburst, observed at other dwarf-nova outbursts. However, the source was not covered shortly after the onset of the outburst, so a possible X-ray precursor would have been missed. The soft X-ray outburst ended 1.5 to 2.5 days before the optical outburst had returned to its quiescent level. This is consistent with other simultaneous observations of the decay of dwarf-nova outbursts at different wavelengths. These observations show an earlier return to the quiescent level at shorter wavelengths.