The Allen Telescope Array Twenty-Centimeter Survey—a 700-Square-Degree, Multi-Epoch Radio Data Set. Ii. Individual Epoch Transient Statistics

We present our second paper on the Allen Telescope Array Twenty-centimeter Survey (ATATS), a multi-epoch, ∼700 deg 2 radio image and catalog at 1.4 GHz. The survey is designed to detect rare, bright transients as well as to commission the ATA's wide-field survey capabilities. ATATS explores the challenges of multi-epoch transient and variable source surveys in the domain of dynamic range limits and changing ( u , v ) coverage. Here, we present images made using data from the individual epochs, as well as a revised image combining data from all ATATS epochs. The combined image has rms noise σ = 3.96 mJy beam −1 , with a circular beam of 150'' FWHM. The catalog, generated using a false detection rate algorithm, contains 4984 sources and is >90% complete to 37.9 mJy. The catalogs generated from snapshot images of the individual epochs contain between 1170 and 2019 sources over the 564 deg 2 area in common to all epochs. The 90% completeness limits of the single-epoch catalogs range from 98.6 to 232 mJy. We compare the catalog generated from the combined image to those from individual epochs, and from the NRAO VLA Sky Survey, a legacy survey at the same frequency. We are able to place new constraints on the transient population: fewer than 6 × 10 −4 transients deg −2 , for transients brighter than 350 mJy with characteristic timescales of minutes to days. This strongly rules out an astronomical origin for the ∼1 Jy sources reported by Matsumura et al. based on their stated rate of 3.1 × 10 −3 deg −2 .