Unexplained Hard X-ray Emission from Hot Single White Dwarfs

The central star of the Helix Nebula, WD2226210, is a hot white dwarf (WD). ItsXray emission has a soft photospheric component and a harder component peakingnear 1 keV, which cannot be explained, as it does not have a binary companion orfast stellar wind. Only one other single WD, KPD0005+5106, shows similar Xraycharacteristics unambiguously, while two PG1159type WDs have tentativedetections of hard Xrays. We request 300 ks XMM observations of WD2226, usingRGS spectra to determine the temperature, abundances, and ionization equilibriumof the Xrayemitting plasma, and using EPIC data to study the temporalvariations of the 1 keV emission. We also request short observations of theother three WDs to characterize their hard Xray emission, and assess its physical implications. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]