A ROSAT WFC observation of SW UMa: The EUV behaviour of dwarf novae in superoutburst explained

Abstract

During re-processing and analysis of the entire ROSAT Wide Field Camera (WFC) pointed observations data base, we discovered a serendipitous, off-axis detection of the cataclysmic variable SW UMa at the onset of its 1997 October superoutburst. Although long outbursts in this SU UMa-type system are known to occur every ∼ 450 d, none had ever been previously observed in the extreme ultra-violet (EUV) by ROSAT. The WFC observations began just ≈ 13 hr after the optical rise was detected. With a peak count rate of ∼ 4.5 count s-1 in the S1 filter, SW UMa was temporarily the third brightest object in the sky in this waveband. Over the next ≈ 19 hr the measured EUV flux dropped to < 2 count s-1, while the optical brightness remained essentially static at mv ∼ 11. Similar 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 EUV dip during normal outbursts. Therefore, this feature may be common in superoutbursts of SU UMa-like systems. We expand on ideas first put forward by Osaki and Mauche & Raymond and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities that affect the accretion discs in these systems.