GALEX Absolute Calibration and Extinction Coefficients Based on White
  Dwarfs
  We use 1837 DA white dwarfs with high signal to noise ratio spectra and Gaia
parallaxes to verify the absolute calibration and extinction coefficients for
the Galaxy Evolution Explorer (GALEX). We use white dwarfs within 100 pc to
verify the linearity correction to the GALEX data. We find that the linearity
correction is valid for magnitudes brighter than 15.95 and 16.95 for the Far
Ultraviolet (FUV) and Near Ultraviolet (NUV) bands, respectively. We also use
DA white dwarfs beyond 250 pc to calculate extinction coefficients in the FUV
and NUV bands; $R_{\rm FUV}=8.01 \pm 0.07$ and $R_{\rm NUV}=6.72 \pm 0.04$.
These are consistent with the predicted extinction coefficients for Milky Way
type dust in the FUV, but smaller than predictions in the NUV. With well
understood optical spectra and state-of-the-art model atmosphere analysis,
these white dwarfs currently provide the best constraints on the extinction
coefficients for the GALEX data.
