X-Ray Spectra of Cataclysmic Variables from ROSAT

Details X-Ray Spectra of Cataclysmic Variables from ROSAT X-Ray Spectra of Cataclysmic Variables from ROSAT

May 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996apj...462..404r&link_type=abstract

Astrophysical Journal v.462, p.404

Astronomy and Astrophysics

Astronomy

51

Stars: Novae, Cataclysmic Variables, Radiation Mechanisms: Nonthermal, X-Rays: Stars

Scientific paper

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.

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