X-ray observations of a large sample of cataclysmic variable stars using the Einstein Observatory

This paper presents the results of an X-ray survey of 31 known or suspected cataclysmic variables. Eighteen of these close binary systems are detected with inferred luminosities in the 0.1 - 4.0 keV band of between 10³⁰ and 10³²erg s^-1. The majority have relatively hard X-ray spectra (kT > 2 keV) irrespective of luminosity state. Of seven dwarf novae observed during optical outbursts only U Gem exhibited enhanced ultrasoft X-ray emission (kT ≡ 10 eV) in addition to weak, hard X-ray emission. Variability of the X-ray flux is observed in many of these stars, on time-scales ranging from tens of seconds to hours. The contribution to the flux from extended X-ray emission is investigated for SU UMa and GK Per. Several possibilities for the origin of the hard X-rays are considered.