Massive White Dwarf with a Dark Spot on its Surface

Kilic et al. (2015) present the discovery of eclipse-like events around the massive white dwarf J1529+2928. The eclipse-like events occur every 38 minutes and they are believed to be caused by the presence of a dark spot on the surface of the white dwarf that comes into view every 38 minutes due to the rotation of the white dwarf. J1529+2928 is modelled to have a temperature of 11600 K, a radius of 5500 km (i.e. slightly smaller than Earth) and have about the same mass as the Sun. The eclipse-like events cannot be due to a transiting planet because a planet in a 38 minute orbit around the white dwarf would be tidally disrupted as it is too close-in.

Radial velocity measurements of J1529+2928 also show that there is no companion star or companion brown dwarf in orbit around it that could be responsible for the eclipse-like events. The dark spot on J1529+2928 is predicted to have a temperature of roughly 10000 K and covers ~14 percent of the surface area of the white dwarf. The presence of such a dark spot is most likely due to channelling of accreted heavy elements onto a spot on the surface of the white dwarf by a magnetic field. Because heavy elements are more opaque to shorter wavelengths of light, this can explain why the depth of the eclipse-like events is shallower when J1529+2928 is observed at longer wavelengths.

Reference:
Kilic et al. (2015), “A Dark Spot on a Massive White Dwarf”, arXiv:1511.07320 [astro-ph.SR]