Recent Science Highlights

1 November 1995

(the following information provided by Dr. Mark Hurwitz)

EUVE spectrometer observations of the eclipsing DQ Her star EX Hya will be presented at the January AAS meeting by Mark Hurwitz, Martin Sirk, and Stuart Bowyer:

• This system shows strong flux modulation at both the binary period (5895s) and the white dwarf (WD) spin period (4022s). 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 Angstroms, or a column density (assuming neutral material) of about 1.3E20 cm^-2. The flux during the eclipse from the secondary star is consistent with zero. This eclipse is very short (~40s) and confines the EUV bright region to within a few WD radii of the primary. 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 about 10 narrow emission lines primarily from highly ionized Iron. The EUV spectrum is very similar to that of HR 1099, suggesting a peak temperature of a few times 10^7 K.