Cataclysmic Variables: Temporal and Spectral Studies

Here we present long-term temporal and X-ray spectral studies of cataclysmic variables (CVs). Recent interest in long periods which may be present in CVs has been motivated principally by the idea that solar-like magnetic cycles might be important. In Chapter 2, we test the presence of such periods in light curves compiled from amateur visual observations for four well-studied stars, and evaluate the case for long-term periods in other CVs. We find that none of the stars studied to date exhibits a good case for strictly periodic variability. Nevertheless, the observed amplitudes (0.2 mag) and apparent time scales (5-40 yrs) of variability are plausible consequences from solar-like magnetic cycles. We extend a theory of P_orb changes to the case of accretion-powered binaries, and show that it predicts accretion rate variations consistent with observation. The proximate cause of the P_orb and M variations is the cyclic transfer of angular momentum to and from the outer layers of the contact star. The underlying cause of these phenomena is solar-type cycles of the contact star. In Chapter 3, we determine X-ray spectral parameters and the orbital dependence for 37 disk-accreting CVs observed with the ROSAT Position Sensitive Proportional Counter. The raw data are fit with an optically thin, thermal bremsstrahlung model plus absorption and our findings suggest that the X-ray emission in CVs is not well described by this single temperature model. We address the problem of understanding the observed 0.1-2.4 keV emission. We show that the ratio of X-ray flux to visual flux correlates with accretion rate, absolute visual magnitude and equivalent width of the Hβ emission line. The results are consistent with the X-rays originating in the boundary layer between the accretion disk and the white dwarf. Changes in softness ratio as a function of orbital phase are seen in 17 of the 37 CVs. In particular, the observation of WZ Sge yields strong evidence of a recurrent dip in the soft X-ray flux. In Chapter 4, we investigate the dip exhibited by WZ Sge and find that it is accompanied by a variation in column density around the orbit by a factor of ~7. We argue that the dips are caused by photoelectric absorption in material with a column density of NH ~4 times 10²⁰ cm^{ -2}. Analysis of archival data from the Einstein IPC and from EXOSAT LE/CMA demonstrates that this absorption feature is variable. The similarity of the dip activity exhibited by WZ Sge to that seen in the other CVs 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.