摘要：We analyze extensive ground- and space-based ultraviolet–infrared observations of the variable white dwarf BOKS53856. Photometry and spectroscopy were obtained from the Hubble Space Telescope, Spitzer Space Telescope,Kepler and Swift satellites, Palomar Observatory, and Kitt Peak National Observatory. Time series data spanningthe entire four-year Kepler mission are used to refine the photometric rotation period of the white dwarf to0.2557292(9)days. The associated periodic variability, with full (minimum to maximum) amplitudes of ≈5% and≈10% in the optical and ultraviolet, respectively, is consistent with the presence of several dark spots on the whitedwarf surface. A fit to the observed ultraviolet–infrared spectral energy distribution of BOKS 53856 using a grid ofnon-LTE synthetic spectra gives best values of effective temperature, T WD =31,056(19) K, surface gravity, logg=7.913(5), mass, M WD =0.603(2)M , radius, R WD =0.0142(4) R , reddening, E B–V =0.0533(1), anddistance, d=277(10) pc. The Hubble ultraviolet spectrum does not show a detectable signature of accreted metals,and the Spitzer infrared photometry does not reveal an excess attributable to a circumstellar dust disk. Nonetheless,the ground-based optical spectra show a very weak, variable Ca II K absorption line that is phased with the Keplerlight curve, being strongest at minimum brightness. We discuss BOKS 53856 and similar objects in the context ofa localized enhancement of absorption from metals in dark (possibly magnetic) spots on the white dwarf surface.
