X-Ray Spectra of Cataclysmic Variables from ROSAT

For 37 disk-accreting cataclysmic variables (CVs) observed with the ROSAT Position Sensitive Proportional Counter, we determine X-ray spectral parameters and the orbital dependence thereof. The raw data are fitted with an optically thin, thermal bremsstrahlung model plus absorption. We obtain average values for the temperature kT ∼ 2.4 keV and column densities NH ∼ 5 x 1020 cm-2 Estimated 0.1-2.4 keV fluxes are in the range log FX = - 13.5 to - 11 ergs cm-2 s-1. Systematic excesses in the residuals of the fitted spectra near E = 0.2 and 0.8 keV suggest that the X-ray emission in CVs is not well described by this single-temperature model. Changes in softness ratio as a function of orbital phase are seen in 17 of the 37 CVs. Our results typically exhibit a decrease in the 0.1-0.4 keV flux near orbital phases (φ)= 0.1 and (φ) = 0.5. Variability in spectral hardness on several timescales is seen in three systems. 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. A coronal contribution from the companion to the X-ray luminosity is rejected because the luminosities found are above the saturation boundary in LX determined for late-type main-sequence stars.