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Astron. Astrophys. 349, 588-594 (1999)

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1. Introduction

X-rays from dwarf novae arise very near the white dwarf, presumably in a boundary layer between the white dwarf and the accretion disk surrounding it. Information on the properties of the X-ray emitting gas as a function of the mass transfer rate through the accretion disk is provided by observations through the outburst cycle of dwarf novae. It may be hoped that such observations help to elucidate the nature of the X-ray emission in cataclysmic variables, and by extension in accretion disks in general.

VW Hyi is a dwarf nova that has been extensively studied during outbursts and in quiescence, at wavelengths from optical to hard X-rays. It is a dwarf nova of the SU UMa type, i.e. in addition to ordinary dwarf nova outbursts it occasionally shows brighter and longer outbursts, which are called superoutbursts. Ordinary outbursts of VW Hyi occur every 20-30 d and last 3-5 days; superoutbursts occur roughly every 180 d and last 10-14 d (Bateson 1977).

A multi-wavelength campaign combining data obtained with EXOSAT, Voyager, the International Ultraviolet Explorer, and by ground based optical observers covered three ordinary outbursts, one superoutburst, and the three quiescent intervals between these outbursts (Pringle et al. 1987, Van Amerongen et al. 1987, Verbunt et al. 1987, Polidan & Holberg 1987, van der Woerd & Heise 1987). The EXOSAT data show that the flux in the 0.05-1.8 keV range decreases during the quiescent interval; the flux evolution at lower energies and at higher energies (1-6 keV) are compatible with this, but the count rates provided by EXOSAT are insufficient to show this independently. Folding the EXOSAT data of three outbursts showed that a very soft component appears early in the outbursts and decays faster than the optical flux (Wheatley et al. 1996).

The ROSAT Position Sensitive Proportional Counter (PSPC) and Wide Field Camera (WFC) covered a dwarf nova outburst of VW Hyi during the ROSAT All Sky Survey (Wheatley et al. 1996). The PSPC data show that the flux in the 0.1-2.5 keV range is lower during outburst. The ROSAT data showed no significant difference between outburst and quiescent X-ray spectrum. The best spectral constraints are obtained for the quiescent X-ray spectrum by combining ROSAT WFC from the All Sky Survey with data from ROSAT PSPC and GINGA pointings. A single temperature fit is not acceptable, the sum of two optically thin plasma spectra, at temperatures of 6 keV and 0.7 keV is somewhat better. The spectrum of a plasma which cools from 11 keV and has emission measures at lower temperatures proportional to the cooling time, provides an acceptable fit of the spectrum in the 0.05-10 keV energy range (Wheatley et al. 1996).

In this paper we report on a series of BeppoSAX observations of VW Hyi, which cover an ordinary outburst and a substantial part of the subsequent quiescent interval. The observations and data reduction are described in Sect. 2, the results in Sect. 3 and a discussion and comparison with earlier work is given in Sect. 4.

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© European Southern Observatory (ESO) 1999

Online publication: September 2, 1999