VLA FRB May 2014 Report

Casey J. Law on behalf of the VLA FRB collaboration


We report on the state of observing for the VLA FRB project, also known as 13B-409 and 14A-425. We detected no FRBs in the first 76 hours observed under an approved DDT proposal (13B-409). Comparing our rate constraint to published rates revealed inconsistencies in published rates that led to an overestimate of the chance for a VLA detection. We discuss an improved rate estimate and find that completing the 147-hour campaign under program 14A-425 will give us a 50% chance at detecting an FRB. The completed VLA observations are the first interferometric search for FRBs and have already highlighted the value of using an interferometer to define a robust FRB rate limit. Ensuring that 14A-425 is observed to completion will significantly improve our chances of making the first interferometric detection of an FRB; an extremely exciting scientific result.

Current Status

Observations took place between late September 2013 and mid January 2014 (see Table \ref{fields}). The array was in CnB configuration for the first 10 hours observed, was being reconfigured during the next 10 hours of observing, and was in B configuration for the final 56 hours observed. We observed for a total of 76 hours and were on our target fields for 63.3 hours for an observing efficiency of 83%.

Fields for September 2013 to January 2014 FRB Campaign
Name RA Dec Time
(J2000) (J2000) (hrs)
RA02 2:27:52.7 +9:13:24.3 2
CDF-South 3:32:28.0 –27:48:30.0 4
RA05 5:04:37.0 –30:50:0.1 16
COSMOS 10:00:28.6 +2:12:21.0 10
RA12 12:00:7.2 +5:53:12.0 4
FRB120127 23:15:00 –18:25:00 40


All 63.3 hours of time on extragalactic fields has been searched for transients with dispersion measures from 0 to 3000 pc cm\(^{-3}\) at a timescale of 5 ms. Figure \ref{snrhist} shows the typical SNR histogram of candidates greater than \(6.5\sigma\), which are saved for analysis. Nearly all candidates are consistent with thermal noise. Eight candidates deviated slightly from the thermal noise distribution and were inspected in detail. All of these candidates were found to be affected by RFI or were highly sensitive to flagging or imaging parameters.

Our analysis shows that we can exclude the presence of astrophysical transients on timescales of 5 milliseconds and below. We measured data quality at regular intervals throughout the search and found that roughly 1% of images had noise that was more than twice the median image noise. Our flux-calibrated observations have a median image noise of 12–14 mJy, as expected for 5-ms, L-band images made with data from 26 good antennas and 230 MHz of bandwidth. To include variance in the noise measurements, we define a 96% completeness for a \(1\sigma\) image sensitivity of 15 mJy or an \(8\sigma\) flux limit of 120 mJy. Observations of pulsar B0355+54 at a range of offset positions shows that imaging sensitivity scales as expected for the VLA primary beam gain pattern. This end-to-end test also confirms that our transient search pipeline works as expeted.

Figure \ref{rate_pub} summarizes the published FRB event rates and the VLA rate limit to FRBs shorter than our integration time of 5 ms. In constructing this figure, we discovered that the sensitivity of published surveys are defined inconsistently. Spitler et al. (2014) calculate the mean beam gain within the FWHM. Burke-Spolaor & Bannister (2014, submitted; hereafter “BSB14”) use half the main beam gain. Lorimer et al. (2007) use the measured fluence of their detection to define a fluence limit. Finally, Thornton et al. (2013) don’t report a fluence limit at all, but instead measure the mean fluence of all detections. For this figure, we use the mean primary beam gain, as in Spitler et al. (2014), although this clearly overestimates the sensitivity at the half-power point, as demonstrated in our pulsar tests.

Cumulative histogram of SNR for candidates in all observations in January 2014. Candidates with \(SNR>6.5\) are shown on a log scale, which shows a noise-like distribution as a linear decline. \label{snrhist}

FRB rates and current VLA rate limit as a function of limiting fluence as quoted in publications (Lorimer et al., 2007; Thornton et al., 2013; Spitler et al., 2014, BSB14). The blue line shows an extrapolation of the rate of Thornton et al. (2013), assuming a Euclidean distribution (\(-3/2\) powerlaw slope in this space). The current VLA 95% upper limit on the FRB event rate is shown with a red triangle and the red cross shows 50% limit (equivalent to one expected detection) is equal to the extrapolated Thornton et al. (2013) rate. Note that the fluence limit of most of these surveys are calculated with different assumptions. \label{rate_pub}

Revised FRB Rate

Existing FRB rates were estimated with order of magnitude precision. These approximations were complicating our ability to estimate the chance of success for the VLA, particularly since the VLA is more sensitive than Parkes and we use a scaling law to infer our chance of success. Therefore, we developed a new, homogeneous system for measuring the flux limit based on the radiometer equation. This approach uses the known Galactic brightness distribution, dispersion, and scattering to include effects known to hinder detectability, as described in BSB14. The result is a calculation of the mean flux limit for each survey assuming that the pulses are cosmological and are dispersed and scattered by the Galaxy before being detected.

Figures \ref{recalcrate5} and \ref{recalcrate1} show the measured FRB rate for all four publications with detections for assumed pulse widths of 5 and 1 ms, respectively. The revised flux limits bring all four published rates into agreement over this range of assumed pulse widths. These estimates converge to an overall rate about 3 times smaller than typically inferred from Thornton et al. (2013), since that rate limit was defined for a mean detected fluence, not a fluence limit. This fact has been widely overlooked and we are including a discussion of this fact in our coming paper on the VLA FRB project.

BSB14 and Petroff et al. (2014) have found a significant deficit of FRB detections at low Galactic latitudes, suggesting that FRBs originate from beyond the Galaxy. All detections in Figures \ref{recalcrate5} and \ref{recalcrate1} have been made in regions with relatively low Galactic dispersion (either high latitude or outer Galaxy), so even before correction for Galactic effects, these rates are close approximations to the VLA FRB target fields.

The figures show the range of constraints of the current and full VLA FRB observing.