The X-ray and extreme-ultraviolet flux evolution of SS Cygni throughout outburst |
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Authors: | Peter J. Wheatley Christopher W. Mauche Janet A. Mattei |
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Affiliation: | Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH;Lawrence Livermore National Laboratory, L-473, 7000 East Avenue, Livermore, CA 94550, USA;American Association of Variable Star Observers, 25 Birch Street, Cambridge, MA 02138-1205, USA |
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Abstract: | We present the most complete multiwavelength coverage of any dwarf nova outburst: simultaneous optical, Extreme Ultraviolet Explorer and Rossi X-ray Timing Explorer observations of SS Cygni throughout a narrow asymmetric outburst. Our data show that the high-energy outburst begins in the X-ray waveband 0.9–1.4 d after the beginning of the optical rise and 0.6 d before the extreme-ultraviolet rise. The X-ray flux drops suddenly, immediately before the extreme-ultraviolet flux rise, supporting the view that both components arise in the boundary layer between the accretion disc and white dwarf surface. The early rise of the X-ray flux shows that the propagation time of the outburst heating wave may have been previously overestimated. The transitions between X-ray and extreme-ultraviolet dominated emission are accompanied by intense variability in the X-ray flux, with time-scales of minutes. As detailed by Mauche & Robinson, dwarf nova oscillations are detected throughout the extreme-ultraviolet outburst, but we find they are absent from the X-ray light curve. X-ray and extreme-ultraviolet luminosities imply accretion rates of 3 × 1015 g s−1 in quiescence, 1 × 1016 g s−1 when the boundary layer becomes optically thick, and ∼1018 g s−1 at the peak of the outburst. The quiescent accretion rate is two and a half orders of magnitude higher than predicted by the standard disc instability model, and we suggest this may be because the inner accretion disc in SS Cyg is in a permanent outburst state. |
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Keywords: | accretion, accretion discs binaries: close stars: dwarf novae stars: individual: SS Cygni novae, cataclysmic variables |
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