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In a dwarf nova in outburst, the central white dwarf is believed to accrete material via an accretion disk which extends down to the white dwarf surface, via a boundary layer. In quiescence, accretion via the boundary layer may continue at a lower rate; in addition, a recent theory predicts the presence of a hot, extended corona which may be partially responsible for the hard (2--10 keV) X-rays observed in quiescent dwarf novae.
The most direct way to distinguish between a compact boundary layer and an extended corona is the observations of hard X-ray eclipses: the former will produce a narrow, deep eclipse while the latter will result in a wide, partial one. We have selected HT Cas as our target for this experiment, because this it the brightest known hard X-ray source among eclipsing dwarf novae. Our previous attempt using the ROSAT satellite was only partially successful, as the system was found in an unusual low state and only 2 eclipses were successfully observed under favorable conditions. Here we present the preliminary results from our ASCA observation of HT Cas, obtained on 1994 Sep 6/7 UT, when the system was in normal quiescence.
