HARD X-RAY EMISSION ASSOCIATED WITH WHITE DWARFS. II

Inspired by the hard X-ray emission from WD 2226-210, the central star of the Helix Nebula, we have made a systematic search for similar sources by correlating the white dwarf catalog of McCook & Sion and the ROSAT Position Senstive Proportional Counter (PSPC) point-source catalog of White, Giommi, & Angelini. We find 76 white dwarfs coincident with X-ray sources at a high level of confidence. Among these sources, 17 show significant hard X-ray emission at energies greater than 0.5 keV. Twelve of these white dwarfs with hard X-ray emission are in known binary systems, in two of which the accretion of the close companion's material onto the white dwarf produces hard X-ray emission, and in the other 10 the late-type companions' coronal activity emits hard X-rays. One apparently single white dwarf is projected near an active galactic nucleus that is responsible for the hard X-ray emission. The remaining four white dwarfs and two additional white dwarfs with hard X-ray emission appear single. The lack of near-IR excess from the apparently single white dwarfs suggests that either X-ray observations are more effective than near-IR photometry in diagnosing faint companions, or a different emission mechanism is needed. It is intriguing that 50% of the six apparently single white dwarfs with hard X-ray emission are among the hottest white dwarfs. We have compared X-ray properties of 11 hot white dwarfs with different spectral types and conclude that stellar pulsation and fast stellar winds are not likely the origin of the hard X-ray emission, but a leakage of the high-energy Wien tail of emission from deep in the stellar atmosphere remains a tantalizing source of hard X-ray emission from hot DO and DQZO white dwarfs. A complete survey using the entire ROSAT PSPC archive is needed to enlarge the sample of white dwarfs with hard X-ray emission. Follow-up near-IR photometric observations are needed to verify the existence of late-type companions, and high-resolution deep X-ray observations are needed to verify the positional coincidence and to study the X-ray spectral properties in order to determine the origin and nature of the hard X-ray emission.