Hard X-Ray Emission Associated with White Dwarfs. IV. Signs of Accretion from Substellar Companions

KPD 0005+5106, with an effective temperature of $\simeq$200,000 K, is one the hottest white dwarfs (WDs). ROSAT unexpectedly detected "hard" ($\sim$1 keV) X-rays from this apparently single WD. We have obtained Chandra observations that confirm spatial coincidence hard X-ray source 0005+5106. also XMM-Newton as well PG 1159$-$035 and WD 0121$-$756, which are whose were by at 3$\sigma$-4$\sigma$ levels. The spectra three WDs show remarkably similar shapes can be fitted models including a blackbody component for stellar photospheric emission, thermal plasma emission component, power-law component. Their luminosities in $0.6-3.0$ keV band range $4\times10^{29}$ to $4\times10^{30}$ erg~s$^{-1}$. EPIC-pn soft-band ($0.3-0.5$ lightcurve 0005+5106 essentially constant, but hard-band ($0.6-3.0$ shows periodic variations. An analysis generalized Lomb-Scargle periodograms lightcurves finds convincing modulation (false alarm probability 0.41%) period 4.7$\pm$0.3 hr. Assuming corresponds binary orbital period, Roche radii viable types companion been calculated: M9V star, T brown dwarf, Jupiter-like planet. Only planet has size larger than its radius, although star dwarf may heated inflate past radius. Thus, all donors fuel accretion-powered emission.