A ROSAT WFC observation of SW UMa: the EUV behaviour of dwarf novae in superoutburst explained |
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Authors: | MR Burleigh JP Pye SW Poulton KB Sohl PJ Wheatley GA Wynn |
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Institution: | Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH |
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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 m 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, 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. |
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Keywords: | accretion accretion discs binaries: close stars: individual: SW UMa novae cataclysmic variables ultraviolet: stars |
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