X-Ray Emission from Cataclysmic Variables with Accretion Disks - Part Two - Extreme Ultraviolet Soft X-Ray Radiation

Abstract

About half of the gravitational luminosity released by gas accreting onto a white dwarf through a disk should emerge from the star/disk boundary layer. For the accretion rates present in many cataclysmic variables, theory predicts that this luminosity should be in the form of an optically thick EUV/soft X-ray component, with Te ≍ (1-3) x 105 K. We compare the theoretical predictions with presently available soft X-ray observations and find satisfactory agreement. Previous doubts on this point were based on inappropriate choices for several critical parameters: white dwarf mass, interstellar column density, and the space density of classical novae. We also attempt to constrain the boundary layer radiation by comparing observed and predicted strengths of the He II lambda 1640 and lambda 4686 emission lines, assuming that these are produced by photoionization in the upper layers of the disk. The results support the simple optically thick model for high-A systems, but may require complicated X-ray spectra in low-M systems.