The GALEX Time Domain Survey I. Selection and Classification of Over a
  Thousand UV Variable Sources
  We present the selection and classification of over a thousand ultraviolet
(UV) variable sources discovered in ~ 40 deg^2 of GALEX Time Domain Survey
(TDS) NUV images observed with a cadence of 2 days and a baseline of
observations of ~ 3 years. The GALEX TDS fields were designed to be in spatial
and temporal coordination with the Pan-STARRS1 Medium Deep Survey, which
provides deep optical imaging and simultaneous optical transient detections via
image differencing. We characterize the GALEX photometric errors empirically as
a function of mean magnitude, and select sources that vary at the 5\sigma level
in at least one epoch. We measure the statistical properties of the UV
variability, including the structure function on timescales of days and years.
We report classifications for the GALEX TDS sample using a combination of
optical host colors and morphology, UV light curve characteristics, and matches
to archival X-ray, and spectroscopy catalogs. We classify 62% of the sources as
active galaxies (358 quasars and 305 active galactic nuclei), and 10% as
variable stars (including 37 RR Lyrae, 53 M dwarf flare stars, and 2
cataclysmic variables). The remaining unclassified sources include UV-bright
extragalactic transients, two of which have been spectroscopically confirmed to
be a young core-collapse supernova and a flare from the tidal disruption of a
star by dormant supermassive black hole. We calculate a surface density for
variable sources in the UV with NUV < 23 mag and |\Delta m| > 0.2 mag of ~ 8.0,
7.7, and 1.8 deg^-2 for quasars, AGNs, and RR Lyrae stars, respectively. We
also calculate a surface density rate in the UV for transient sources, using
the effective survey time at the cadence appropriate to each class, of ~ 15 and
52 deg^-2 yr^-1 for M dwarfs and extragalactic transients, respectively.
(Abridged.)
