GD 394: A mystery at all wavelengths

Recent discoveries have demonstrated that planetary systems routinely survive the post-main-sequence evolution of their host stars, leaving the resulting white dwarf with a rich circumstellar environment. Among the most intriguing is the hot white dwarf GD 394, exhibiting a unique 1.150 ± 0.003 day flux variation detected in Extreme Ultraviolet Explorer observations in the mid-1990s. The variation has eluded a satisfactory explanation, but hypotheses include channelled accretion producing a dark spot of metals, occultation by a gas cloud from an evaporating planet, or heating from a flux tube produced by an orbiting iron-cored planetesimal. We present TESS observations of GD 394 demonstrating a 0.12 ± 0.01% flux variation with a period of 1.146 ± 0.001 days, consistent with the EUVE period and the first re-detection of the flux variation outside of the extreme ultraviolet. We describe the multi-wavelength observational history of GD 394, including HST, TESS and Chandra data, and discuss the implications of our results for the various physical explanations put forward for the variability of GD 394 and the opportunities to discover more GD394-like white dwarfs with TESS in the future.