1FGL J0523.5-2529: A New Probable Gamma-ray Pulsar Binary
  We report optical photometric and SOAR spectroscopic observations of an X-ray
source found within the localization error of the Fermi-LAT unidentified
gamma-ray source J0523.5-2529. The optical data show periodic flux modulation
and radial velocity variations indicative of a binary with a 16.5-hr period.
The data suggest a massive non-degenerate secondary (~> 0.8 M_sun), and we
argue the source is likely a pulsar binary. The radial velocities have good
phase coverage and show evidence for a measurable eccentricity (e=0.04). There
is no clear sign of irradiation of the secondary in either photometry or
spectroscopy. The spatial location out of the Galactic plane and gamma-ray
luminosity of the source are more consistent with classification as a recycled
millisecond pulsar than as a young pulsar. Future radio timing observations can
confirm the identity of the primary and further characterize this interesting
system.
