Hard X - ray and Infrared Emission from Apparently Single

The photospheric emission of a white dwarf (WD) is not expected to be detectable in hard X-rays or the mid-IR. Hard X-ray (∼1 keV) emission associated with a WD is usually attributed to a binary companion; however, emission at 1 keV has been detected from three WDs without companions: KPD 0005+5106, PG 1159, and WD 2226−210. The origin of their hard X-ray emission is unknown, although it has been suggested that WD 2226−210 has a late-type companion whose coronal activity is responsible for the hard X-rays. Recent Spitzer observations of WD 2226−210 revealed mid-IR excess emission indicative of the existence of a dust disk. It now becomes much less clear whether WD 2226−210's hard X-ray emission originates from the corona of a late-type companion or from the accretion of the disk material. High-quality X-ray observations and mid-IR observations of KPD 0005+5106 and PG 1159 are needed to help us understand the origin of their hard X-ray emission. White dwarfs (WDs) can be associated with three types of X-ray sources: (1) photospheric X-ray emission from the WD itself, (2) accretion of material from a close binary companion, as in cataclysmic variables, and (3) coronal X-ray emission from a late-type binary companion, such as dMe stars. The latter two types of sources originate from WDs in binary systems and commonly show X-ray emission at 1 keV. In contrast, the photospheric X-ray emission from WDs is much softer, well below 0.5 keV. No hard X-ray emission is expected from single WDs. Inspired by the hard X-ray emission from WD 2226−210, the central star of the Helix Nebula (Guerrero et al. 2001), we made a systematic search for hard X-ray emission from WDs using the WD catalog compiled by McCook & Sion (1999) and the Second ROSAT Source Catalogs of Pointed Observations with the PSPC with and without the Boron Filter (2RXF and 2RXP). Among the 124 X-ray sources that are found coincident with WDs, 22 show hard X-ray emission peaking near 0.8-0.9 keV. After identifying binary systems and extraneous X-ray sources, we find two WDs, KPD 0005+5106 (= WD 0005+511) and PG 1159 (= WD 1159−034), that are associated with hard X-ray emission and appear to be single (O'Dwyer et al. 2003; Chu et al. 2004a).