The Allen Telescope Array Pi GHz Sky Survey II. Daily and Monthly
  Monitoring for Transients and Variability in the Bootes Field
  We present results from daily radio continuum observations of the Bootes
field as part of the Pi GHz Sky Survey (PiGSS). These results are part of a
systematic and unbiased campaign to characterize variable and transient sources
in the radio sky. The observations include 78 individual epochs distributed
over 5 months at a radio frequency of 3.1 GHz with a median RMS image noise in
each epoch of 2.8 mJy. We produce 5 monthly images with a median RMS of 0.6
mJy. No transient radio sources are detected in the daily or monthly images. At
15 mJy, we set an upper limit (2 sigma) to the surface density of 1-day radio
transients at 0.025 deg^-2. At 5 mJy, we set an upper limit (2\sigma) to the
surface density of 1-month radio transients at 0.18 deg^-2. We also produce
light curves for 425 sources and explore the variability properties of these
sources. Approximately 20% of the sources exhibit some variability on daily and
monthly time scales. The maximum RMS fractional modulations on the 1 day and 1
month time scales for sources brighter than 10 mJy are 2 and 0.5, respectively.
The probability of a daily fluctuation for all sources and all epochs by a
factor of 10 is less than 10^-4. We compare the radio to mid-infrared
variability for sources in the field and find no correlation. Finally, we apply
the statistics of transient and variable populations to constrain models for a
variety of source classes.
