Relation between the X-ray and optical luminosities in binary systems with accreting nonmagnetic white dwarfs

We investigate the relation between the optical (g-band) and X-ray (0.5-10 keV) luminosities of accreting nonmagnetic white dwarfs. According to the present-day counts of the populations of star systems in our Galaxy, these systems have the highest space density among the close binary systems with white dwarfs. We show that the dependence of the optical luminosity of accreting white dwarfs on their X-ray luminosity forms a fairly narrow one-parameter curve. The typical half-width of this curve does not exceed 0.2-0.3 dex in optical and X-ray luminosities, which is essentially consistent with the amplitude of the aperiodic flux variability for these objects. At X-ray luminosities L (x) similar to 10(32) erg s(-1) or lower, the optical g-band luminosity of the accretion flow is shown to be related to its X-ray luminosity by a factor similar to 2-3. At even lower X-ray luminosities (L (x) a parts per thousand(2) 10(30) erg s(-1)), the contribution from the photosphere of the white dwarf begins to dominate in the optical spectrum of the binary system and its optical brightness does not drop below M (g) similar to 13-14. Using the latter fact, we show that in current and planned X-ray sky surveys, the family of accreting nonmagnetic white dwarfs can be completely identified to the distance determined by the sensitivity of an optical sky survey in this region. For the Sloan Digital Sky Survey (SDSS) with a limiting sensitivity m (g) similar to 22.5, this distance is similar to 400-600 pc.