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  • VLA FRB May 2014 Report

    Casey J. Law on behalf of the VLA FRB collaboration

    Summary

    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

    \label{fields}

    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}