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Session 79 - CVs and Novae.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center
We present high time resolution EUV light curves and phase resolved spectra for the eclipsing DQ Her star EX Hya obtained by NASA's Extreme Ultraviolet Explorer\/ satellite. This system shows strong flux modulation at both the binary period (5895 s) and the white dwarf (WD) spin period (4022 s). A deep eclipse lasting 0.15 of the binary orbital period occurs just prior to phase 0.0 and indicates the presence of a bulge in the accretion disk with an optical depth about 1.8 at 100 Åor a column density (assuming neutral material) of about 1.3 \times 10^20 cm^-2. The flux during the eclipse from the secondary star is consistent with zero. This eclipse is very short (\approx 40 s) and confines the EUV bright region to just a few WD radii. The eclipse profile and centroid do not vary with the WD spin phase, indicating that most of the EUV emission arises on, or very close to, the WD surface as opposed to the inner regions of the accretion disk. The EUV flux varies by a factor 3.7 as a function of WD spin phase. We attribute this to absorption by an accretion curtain that intercepts EUV light most strongly when both the EUV accretion region and the curtain face are most nearly in line with the observer. The EUV modulation at the WD spin period can be successfully reproduced with a simple model of sinusoidally varying optical depth in the curtain. When the source is bright, the EUV spectrum shows between 3 and \approx 10 narrow emission lines from highly ionized iron, similar to the spectra of stellar coronae at a temperature of about 10^7 K\@. This work has been supported by NASA grant NAG5-2623.
