X-ray spectra of cataclysmic variables from the Einstein Observatory

Abstract

We determine X-ray spectral parameters of 32 cataclysmic variables observed with the Einstein imaging proportional counter, by fitting an optically thin, thermal bremsstrahlung spectrum to the final, reprocessed version of the raw data. The conclusions of earlier studies, based on the first processing of the data, are not substantially changed. Most of the stars show temperatures in the range 1-5 keV, while a few exhibit harder spectra with temperatures in excess of 10 keV. Estimated 0.1-3.5 keV luminosities are generally in the range 1030-1032 ergs s-1. The results are consistent with the X-rays originating in a disk/white dwarf boundary layer of nonmagnetic systems, or in a hot, postshock region in the accretion column of DQ Her stars. Contribution to the X-ray luminosity from the corona of the companion is probably negligible as the luminosities found are well above the upper limits recently determined for the X-ray luminosities of late-type main-sequence stars. In a few objects we find column densities that are unusually high for interstellar material, suggesting that the absorption occurs in the system itself, and very likely within the Roche lobe of the primary. Absorption of X-rays in the outer parts of the accretion disk seems to be a plausible explanation, especially since the systems displaying highly absorbed spectra are known to have high inclinations.