The Allen Telescope Array Pi GHz Sky Survey - III: The ELAIS-N1, Coma, and Lockman Hole Fields

TL;DR

This study conducted a large, multi-epoch radio survey at 3.1 GHz over four fields, searching for transient sources and setting upper limits on their occurrence, with no confirmed transients detected.

Contribution

It provides one of the shortest cadence, largest area, multi-epoch surveys at these frequencies, and establishes new upper limits on transient source populations.

Findings

No confirmed transients with >10x variability found.

Detected one source brightening by a factor of six in a single epoch.

Placed upper limits on transient surface densities at various flux levels.

Abstract

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 (PiGSS). The observations were taken approximately once per day between 2009 May and 2011 April. Each image covers 11.8 square degrees and has 100 arcsecond FWHM resolution. Deep images for each of the four fields have rms noise between 180 and 310 uJy 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 10. However, we find one source which 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 of about three 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 / sq. deg. with characteristic timescales of months to years; fewer than 0.02 / sq. deg. with timescales of months; and fewer than 0.009 / sq. deg with timescales of days. We also plot upper limits as a function of flux density for transients on the same timescales.