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 database, 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 days, 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 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 while the optical brightness remained essentially static at V~11. Similar behaviour has also been recently reported in the EUV light curve of the related SU UMa-type binary OY Car during superoutburst (Mauche and Raymond 2000). 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 (1994, 1995) and Mauche and Raymond (2000) and offer an explanation for this behaviour by examining the interplay between the thermal and tidal instabilities which affect the accretion disks in these systems.