White Dwarf based evaluation of the GALEX absolute calibration

Abstract

This paper describes a revised photometric calibration of the \emph{Galaxy Evolution Explorer} magnitudes, based on measurements of DA white dwarfs. The photometric magnitudes of white dwarfs measured by \emph{GALEX} are compared to predicted magnitudes based on independent spectroscopic data (108 stars) and alternately to \emph{IUE} UV fluxes of the white dwarfs (218 stars). The results demonstrate a significant non-linear correlation and small offset between archived \emph{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 \emph{GALEX} magnitudes to an absolute magnitude scale that is linear with respect, and directly related, to the \emph{Hubble Space Telescope} photometric scale. These corrections should be valid for stars dimmer than magnitudes 9.3 and 10.5 in the NUV and FUV respectively, and brighter than magnitude 17.5 in both