4. Comparison with previous X-ray observations
4.1. Time variability
We predict the ROSAT count rates of VW Hyi during outburst and quiescence with the observed BeppoSAX flux from the two-temperature fit (see Table 2). Here we do apply (Polidan et al. 1990) since ROSAT is probably more sensitive to than BeppoSAX. The predicted count rates during outburst and quiescence are 0.31 and . The ROSAT observed count rates are 0.4 and respectively (Belloni et al. 1991; Wheatley et al. 1996). Both predictions appear to be different from the observations by a factor .
From Fig. 2, we observe a decrease in MECS count rate by a factor of during outburst. This is inconsistent with the constant 0.4 observed by the ROSAT PSPC during outburst (Wheatley et al. 1996). Using the LECS data during the outburst in a bandwidth (0.1-1.5 keV) comparable to the ROSAT PSPC we cannot discriminate observationally between a constant flux and the exponential decay observed by the MECS. However, our spectral fits to the data require that the 0.1-2.5 keV flux decreases in tandem with the hard flux. Thus the difference between the ROSAT PSPC and the BeppoSAX MECS lightcurves during outburst may either be due to variations between individual outbursts or to the different spectral bandwidths of the observing instruments. The predicted decay of the count rate significantly exceeds the range allowed by the ROSAT observations of the Nov 1990 outburst.
We interpret the time variability of the count rate shown in Figs. 2 and 3, as a change mainly in the amount of gas in the inner disk that emits keV photons. At the end of the outburst, while the inner disk is still predominantly optically thick, the mass accretion rate onto the white dwarf is decreasing. As a result, the amount of hot optically thin gas drops gradually. This is observed in Fig. 2. The transition to a predominantly optically thin inner disk occurs just before observation 2. As a result the amount of optically thin emitting material in the disk increases strongly. This is shown by the increase of the emission measure of the hot component in Fig. 3, observation 2, which even peaks above the quiescent value. The settling of the accretion rate towards quiescence is shown in Fig. 3, observations 3-6 for both the temperature and the emission measure. In contrast to the emission measure, the temperature of the hot component increases only gradually throughout observations 1-6 as it reflects the slowly decreasing accretion rate rather than the amount of optically thin emitting material in the disk.
4.2. Spectral variability
Both a two-temperature plasma model and a cooling flow model fit the spectrum of our BeppoSAX observations of VW Hyi better than a one-temperature model. The contribution of the cool component lies mainly in the presence of strong Fe-L line emission around 1 keV. The hot component contributes the continuum and the Fe-K line emission at keV. Adding a soft atmospheric component in the form of a eV blackbody model does not improve our fits. This blackbody component, reported by Van der Woerd et al. (1986) and Van Teeseling et al. (1993), is too soft to be detected by BeppoSAX LECS.
Based upon the -values, the BeppoSAX observation of VW Hyi does not discriminate between a continuous temperature distribution (the cooling flow model) and a discrete temperature distribution (the two-component model) of the X-ray emitting region. Wheatley et al. (1996) derive a lower and upper temperature of and keV respectively for a cooling flow fit to the combined ROSAT PSPC and GINGA LAC data during quiescence. These temperatures are consistent with our cooling flow fits to BeppoSAX data during quiescence; there is a small overlap between the 2 and 3 contours shown in Fig. 6 by Wheatley et al. and the contours of our Fig. 6.
© European Southern Observatory (ESO) 1999
Online publication: September 2, 1999