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 deg² 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 deg² 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.