Variable Radio Sources in the Galactic Plane
  Using three epochs of VLA observations of the Galactic Plane in the first
quadrant taken ~15 years apart, we have conducted a search for a population of
variable Galactic radio emitters in the flux density range 1-100 mJy at 6 cm.
We find 39 variable sources in a total survey area of 23.2 sq deg. Correcting
for various selection effects and for the extragalactic variable population of
active galactic nuclei, we conclude there are ~1.6 Galactic sources per sq deg
which vary by more than 50% on a time scale of years (or shorter). We show that
these sources are much more highly variable than extragalactic objects; more
than 50% show variability by a factor >2 compared to <10% for extragalactic
objects in the same flux density range. We also show that the fraction of
variable sources increases toward the Galactic center (another indication that
this is a Galactic population), and that the spectral indices of many of these
sources are flat or inverted. A small number of the variables are coincident
with mid-IR sources and two are coincident with X-ray emitters, but most have
no known counterparts at other wavelengths. Intriguingly, one lies at the
center of a supernova remnant, while another appears to be a very compact
planetary nebula; several are likely to represent activity associated with star
formation regions. We discuss the possible source classes which could
contribute to the variable cohort and followup observations which could clarify
the nature of these sources.
