X-Ray Absorption Dips in WZ Sagittae

A 5 hour ROSAT observation of the recurrent nova WZ Sge has yielded strong evidence of a dip in the soft X-ray flux. The dip is centered at orbital phase phi =0.7 with respect to time of optical eclipse and lasts for about a half an hour. The source's X-ray spectrum can be fitted by a thermal Bremsstrahlung model plus variable absorption; a variation in column density around the orbit by a factor of ~ 7 is observed. We argue that the dips are caused by photoelectric absorption in material with a column density of N_H 5times 10(20) atoms/cm(2) . Analysis of archival data from the Einstein IPC demonstrates that this absorption feature is variable; the average column density in the system was higher ten years earlier but no evidence of increased absorption at phase phi ~ 0.7 is seen. The similarity of the dip activity exhibited by WZ Sge to that seen in other cataclysmic variables and low mass X-ray binaries suggests that they are caused by the same mechanism. The most plausible explanation for the dips is that they result from regular occultation of the X-ray source by a thickened region of an accretion disk at the point where it is fed by the gas stream from a binary companion.