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 similar to 2.4 keV and column densities N-H similar to 5 x 10(20) cm(-2). Estimated 0.1-2.4 keV fluxes are in the range log F-X = -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 phi = 0.1 and phi = 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 H beta 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 L(X) determined for late-type main-sequence stars.