A ROSAT WFC observation of SWUMa: the EUV behaviour of dwarf novae in superoutburst explained

During re-processing and analysis of the entire ROSAT Wide Field Camera (WT C) pointed observations data base, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 Octob er superoutburst. Although long outbursts in this SU UMa-type system are kn own to occur every similar to 450 d, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT The WFC observations began just approximate to 13 hr after the optical rise was detected. With a peak coun t rate of similar to 4.5 count s(-1) in the S1 filter, SW UMa was temporari ly the third brightest object in the sky in this waveband. Over the next ap proximate to 19 hr the measured EUV flux dropped to < 2 count s(-1), while the optical brightness remained essentially static at m(v) similar to 11. S imilar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst, as reported by Mauche & Raymond. In contrast, U Gem-type dwarf novae show no such early EU V dip during normal outbursts. Therefore, this feature may be common in sup eroutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki and Manche & Raymond and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities that af fect the accretion discs in these systems.