Limits on Fast Radio Bursts and other transient sources at 182 MHz using the Murchison Widefield Array

TL;DR

This survey used the Murchison Widefield Array at 182 MHz to search for transient and variable radio sources over timescales from 28 seconds to a year, setting new constraints on their occurrence rates.

Contribution

It provides the most constraining low-frequency limits on transient sources and FRBs, with no detections, refining the understanding of their surface density and rate at 182 MHz.

Findings

No transient candidates detected down to 0.285 Jy.

Set an upper limit of 4.1 x 10^-7 deg^-2 on transient surface density.

Estimated an FRB rate of less than 82 per sky per day at 182 MHz.

Abstract

We present a survey for transient and variable sources, on timescales from 28 seconds to $\sim$1 year, using the Murchison Widefield Array (MWA) at 182 MHz. Down to a detection threshold of 0.285 Jy, no transient candidates were identified, making this the most constraining low-frequency survey to date and placing a limit on the surface density of transients of $<4.1 \times 10^{-7}$ deg$^{-2}$ for the shortest timescale considered. At these frequencies, emission from Fast Radio Bursts (FRBs) is expected to be detectable in the shortest timescale images without any corrections for interstellar or intergalactic dispersion. At an FRB limiting flux density of 7980 Jy, we find a rate of $<$82 FRBs per sky per day for dispersion measures $<$700 pc cm$^{-3}$. Assuming a cosmological population of standard candles, our rate limits are consistent with the FRB rates obtained by Thornton et al. (2013) if they have a flat spectral slope. Finally, we conduct an initial variability survey of sources in the field with flux densities $\gtrsim$0.5 Jy and identify no sources with significant variability in their lightcurves. However, we note that substantial further work is required to fully characterise both the short term and low level variability within this field.