Mysterious Hard X-ray Emission from Apparently Single White Dwarfs

You-Hua Chu^1*, 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, email:yhchu@asiaa.sinica.edu.tw

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