SIMBAD references

We present results from a total of 459 repeated 3.1 GHz radio continuum observations (of which 379 were used in a search for transient sources) of the ELAIS-N1, Coma, Lockman Hole, and NOAO Deep Wide Field Survey fields as part of the Pi GHz Sky Survey. The observations were taken approximately once per day between 2009 May and 2011 April. Each image covers 11.8 square degrees and has 100'' FWHM resolution. Deep images for each of the four fields have rms noise between 180 and 310 µJy, and the corresponding catalogs contain ∼200 sources in each field. Typically 40-50 of these sources are detected in each single-epoch image. This represents one of the shortest cadence, largest area, multi-epoch surveys undertaken at these frequencies. We compare the catalogs generated from the combined images to those from individual epochs, and from monthly averages, as well as to legacy surveys. We undertake a search for transients, with particular emphasis on excluding false positive sources. We find no confirmed transients, defined here as sources that can be shown to have varied by at least a factor of 10. However, we find one source that brightened in a single-epoch image to at least six times the upper limit from the corresponding deep image. We also find a source associated with a z = 0.6 quasar which appears to have brightened by a factor ∼3 in one of our deep images, when compared to catalogs from legacy surveys. We place new upper limits on the number of transients brighter than 10 mJy: fewer than 0.08 transients deg^–2 with characteristic timescales of months to years; fewer than 0.02 deg^–2 with timescales of months; and fewer than 0.009 deg^–2 with timescales of days. We also plot upper limits as a function of flux density for transients on the same timescales.