Relation between the X-ray and Optical Luminosities in Binary Systems with Accreting Nonmagnetic White Dwarfs

TL;DR

This study explores the relationship between optical and X-ray luminosities in accreting nonmagnetic white dwarf binary systems, revealing a narrow correlation useful for identifying such systems in sky surveys.

Contribution

It establishes a quantitative relation between optical and X-ray luminosities, aiding in the detection and study of accreting white dwarf binaries in large sky surveys.

Findings

Optical luminosity correlates with X-ray luminosity within 0.2-0.3 dex.

At Lx~1e32 erg/sec, optical is 2-3 times X-ray luminosity.

White dwarf photosphere dominates optical emission at very low X-ray luminosities.

Abstract

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 Lx~1e32 erg/sec or lower, the optical g-band luminosity of the accretion flow is shown to be related to its X-ray luminosity by a factor ~2-3. At even lower X-ray luminosities (Lx~1e30 erg/sec), 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 Mg~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~22.5, this distance is ~400-600 pc