A pilot survey for transients and variables with the Australian Square Kilometre Array Pathfinder

TL;DR

This pilot survey using ASKAP at 1.4 GHz identified variable AGN/quasars and long-term quasar variability, placing limits on transient occurrence rates, and demonstrating ASKAP's capability for transient and variable source detection.

Contribution

First pilot survey with ASKAP at 1.4 GHz for transients and variables, demonstrating detection of AGN variability and setting upper limits on transient rates.

Findings

Detected nine variable sources with flux variations consistent with scintillation.

Identified a highly variable quasar over a decade, indicating long-term variability.

Placed an upper limit on transient surface density of 0.01 deg$^{2}$ at 95% confidence.

Abstract

We present a pilot search for variable and transient sources at 1.4 GHz with the Australian Square Kilometre Array Pathfinder (ASKAP). The search was performed in a 30 deg$^{2}$ area centred on the NGC 7232 galaxy group over 8 epochs and observed with a near-daily cadence. The search yielded nine potential variable sources, rejecting the null hypothesis that the flux densities of these sources do not change with 99.9% confidence. These nine sources displayed flux density variations with modulation indices m $\geq 0.1$ above our flux density limit of 1.5 mJy. They are identified to be compact AGN/quasars or galaxies hosting an AGN, whose variability is consistent with refractive interstellar scintillation. We also detect a highly variable source with modulation index m $ > 0.5$ over a time interval of a decade between the Sydney University Molonglo Sky Survey (SUMSS) and our latest ASKAP observations. We find the source to be consistent with the properties of long-term variability of a quasar. No transients were detected on timescales of days and we place an upper limit $\rho < 0.01$ deg$^{2}$ with 95% confidence for non-detections on near-daily timescales. The future VAST-Wide survey with 36-ASKAP dishes will probe the transient phase space with a similar cadence to our pilot survey, but better sensitivity, and will detect and monitor rarer brighter events.