ASGARD: A Large Survey for Slow Galactic Radio Transients. I. Overview
  and First Results
  Searches for slow radio transients and variables have generally focused on
extragalactic populations, and the basic parameters of Galactic populations
remain poorly characterized. We present a large 3 GHz survey performed with the
Allen Telescope Array (ATA) that aims to improve this situation: ASGARD, the
ATA Survey of Galactic Radio Dynamism. ASGARD observations spanned 2 years with
weekly visits to 23 deg^2 in two fields in the Galactic Plane, totaling 900 hr
of integration time on science fields and making it significantly larger than
previous efforts. The typical blind unresolved source detection limit was 10
mJy. We describe the observations and data analysis techniques in detail,
demonstrating our ability to create accurate wide-field images while
effectively modeling and subtracting large-scale radio emission, allowing
standard transient-and-variability analysis techniques to be used. We present
early results from the analysis of two pointings: one centered on the
microquasar Cygnus X-3 and one overlapping the Kepler field of view (l =
76{\deg}, b = +13.5{\deg}). Our results include images, catalog statistics,
completeness functions, variability measurements, and a transient search. Out
of 134 sources detected in these pointings, the only compellingly variable one
is Cygnus X-3, and no transients are detected. We estimate number counts for
potential Galactic radio transients and compare our current limits to previous
work and our projection for the fully-analyzed ASGARD dataset.
