Variation of mass accretion rate on to the white dwarf in the dwarf nova VW Hyi in quiescence

Abstract

Abstract We have analysed a series of Suzaku data and one data set of XMM–Newton of the SU UMa type dwarf nova VW Hyi in optical quiescence. The observed spectra in the 0.2–10 keV band are moderately well represented by multitemperature thermal plasma emission models with a maximum temperature of 5–9 keV and bolometric luminosity of (2.4–5.2) × 1030 erg s−1. The mass accretion rate derived from the hard X-ray spectra does not show any clear trend as a function of time since the last superoutburst, in contradiction to theoretical predictions of the disc behaviour of an SU UMa type dwarf nova. The mass accretion rate, on the other hand, shows a clear declining trend with time since the last outburst (including the superoutburst). The rate of decline is of the same order as that evaluated from the hard X-ray light curves of the other two dwarf novae SS Cyg and SU UMa. The standard disc instability model, on the other hand, predicts that the mass accretion rate should increase throughout the optically quiescent phase. We need further observation and theoretical consideration to resolve this discrepancy.