Spin-resolved Far-Ultraviolet Observations of the Magnetic White Dwarf in YY Draconis

Abstract

We observed the intermediate polar YY Draconis with the Far Ultraviolet Spectroscopic Explorer (FUSE), yielding an average far-UV spectrum, as well as spectra and spectrophotometric light curves resolved on the spin phase of the white dwarf in this system. We utilized the BINSYN software suite to calculate model spectra based on a simple system model in which the white dwarf has two identical, two-dimensional, equatorial photospheric hot spots separated by 180° in longitude, which correspond to the poles of a simple centered dipole magnetic field. By optimizing the BINSYN model spectra to match our FUSE data plus archival International Ultraviolet Explorer spectra, we determine a white dwarf effective temperature of 21,500 K with the magnetic pole spots having effective temperatures of 220,000 K and angular radii of 35. Normalization of the model spectra implies a distance to YY Dra of 159 pc. We also construct model light curves resolved on the spin period of the white dwarf via synthetic spectrophotometry of the BINSYN synthetic spectra. These are compared with the light curves constructed from our FUSE data, as well as the Hubble Space Telescope ultraviolet light curves of Haswell et al. In general, we find good agreement between the observed and model spectra and light curves, with the exception of a discrepancy at the shortest wavelengths of the spin-phase minimum light spectrum. This implies the possible presence of vertically extended structure(s) on the white dwarf that are visible above the stellar limb even when the photospheric spots are not and contribute far-UV flux to the observed spectrum that is not reproduced in the BINSYN model. We suggest either the accretion columns above the magnetic poles of the white dwarf or a localized buildup of accreted material on the white dwarf surface at the accretion impact sites as possible sources for the missing far-UV flux. Another possibility invokes a model with a more complex magnetic field geometry, in which the spots are still visible during the spin-phase minimum light configuration, but it is neither conclusive nor well constrained by the available data.