X-Ray Absorption Dips in WZ Sagittae
Session 21 -- Cataclysmic Variables, Dwarf Novae
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

## [21.12] X-Ray Absorption Dips in WZ Sagittae

H.R.\ Richman, J.\ Patterson (Columbia U.)

A 5 hour {\sl 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 $\sim 7$ is observed. We argue that the dips are caused by photoelectric absorption in material with a column density of $N_{\rm H} \sim 5\times 10^{20}$ atoms/cm$^2$. Analysis of archival data from the {\sl 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 $\phi \sim 0.7$ 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.