Mysterious Hard X-ray Emission from Apparently Single White Dwarfs
You-Hua Chu1*, Jesus Toala2, Martin Guerrero3, Florian Bauer3, Jana Bilikova4, Robert Gruendl4
1Astronomy and Astrophysics, Academia Sinica, Taipei, Taiwan
2Institute of Radioastronomy and Astrophysics, UNAM, Morelia, Mexico
3Instituto de Astrofisica de Andalucia, IAA-CSIC, Granada, Spain
4Astronomy, University of Illinois, Urbana, Illinois, USA
* Presenter:You-Hua Chu,
White dwarfs (WDs) by themselves are not supposed to emit X-rays above 0.5 keV; however, a few apparently single WDs are found to be associated with 1 keV X-ray emission. We have obtained Chandra and/or XMM X-ray observations of three apparently single WDs, and confirmed the spatial coincidence of the X-ray sources and WDs. Among the three WDs, KPD0005+5106 has the highest stellar effective temperature (200,000 K), brightest hard X-ray emission, and the longest X-ray observations. We have made X-ray light curves of KPD0005+5106 in soft (0.3-0.5 keV) and hard (0.6-3.0 keV) bands. While the soft X-ray light curve shows a constant level of emission, the hard X-ray light curves show modulations for a period of about 5 hours. Both Chandra and XMM show similar modulations. This light curve and the lack of IR excess can be explained by a substellar companion orbiting around KPD0005+5106 at a distance of 1.2-1.3 solar radii. The companion can be a Jupiter-like planet or a brown dwarf. In either case, the substellar companion's surface is irradiated by the harsh UV and soft X-ray emission from KPD0005+5106, and the boiled off surface material is accreted to the magnetic polar regions of the WD to produce the hard X-ray emission, much like a mild polar.

Keywords: white dwarf, X-ray, accretion, planet, brown dwarf