White-dwarf-based evaluation of the GALEX absolute calibration

This paper describes a revised photometric calibration of the Galaxy Evolution Explorer (GALEX) magnitudes, based on measurements of DA white dwarfs. The photometric magnitudes of white dwarfs measured by GALEX are compared to predicted magnitudes based on independent spectroscopic data (108 stars) and alternately to International Ultraviolet Explorer (IUE) ultraviolet (UV) fluxes of the white dwarfs (218 stars). The results demonstrate a significant non-linear correlation and small offset between archived GALEX fluxes and observed and predicted UV fluxes for our sample. The primary source of non-linearity may be due to detector dead time corrections for brighter stars, but it should be noted that there was a predicted non-linearity in the fainter stars as well. Sample expressions are derived which 'correct' observed GALEX magnitudes to an absolute magnitude scale that is linear with respect, and directly related, to the Hubble Space Telescope photometric scale. These corrections should be valid for stars dimmer than magnitudes 9.3 and 10.5 in the near-UV and far-UV, respectively, and brighter than magnitude 17.5 in both.