The performance and calibration of the CRAFT fly’s eye fast radio burst survey

Abstract

Since January 2017, the Commensal Real-time ASKAP Fast Transients survey (CRAFT) has been utilising commissioning antennas of the Australian SKA Pathfinder (ASKAP) to survey for fast radio bursts (FRBs) in fly's eye mode. This is the first extensive astronomical survey using phased array feeds (PAFs). A total of 23 FRBs have been reported -here, we present a calculation of the sensitivity and total exposure of the survey that detected the first 20 of these bursts, using the pulsars B1641-45 (J1644-4559) and B0833-45 (J0835-4510, i.e. Vela) as calibrators. The design of the survey allows us to benchmark effects due to PAF beamshape, antenna-dependent system noise, radio-frequency interference, and fluctuations during commissioning on timescales from one hour to a year. Observation time, solid-angle, and search efficiency are calculated as a function of FRB fluence threshold. Using this metric, effective survey exposures and sensitivities are calculated as a function of the source counts distribution. Statistical 'stat' and systematics 'sys' effects are treated separately. The implied FRB rate is significantly lower than the 37 sky −1 day −1 calculated using nominal exposures and sensitivities for this same sample by Shannon et al. (2018). At the Euclidean power-law index of −1.5, the rate is 12.7 +3.3 −2.2 (sys) ± 3.6 (stat) sky −1 day −1 above a threshold of 56.6 ± 6.3 (sys) Jy ms, while for the best-fit index for this sample of −2.2, it is 20.7 +2.1 −1.7 (sys) ± 5.8 (stat) sky −1 day −1 above a threshold of 40.4 ± 1.2 (sys) Jy ms. This strongly suggests that these calculations be performed for other FRB-hunting experiments, allowing meaningful comparisons to be made between them.

Limits on absorption from a 332-MHz survey for fast radio bursts

“…However, we can obtain a more reliable constraint on the upper limit of the rate by considering various factors that affect the true rate of FRBs in any given survey. James et al (2019) have shown that it is important to consider the dependence of the sensitivity of the survey on the offset from the boresight of the beam. Depending on the slope of the source count distribution, the effective FoV of the survey will change for some effective flux threshold.…”

Energy function, formation rate, and low-metallicity environment of fast radio bursts

“…As for the beam efficiency for ASKAP, we adopt the best efficiency for the closepack configuration given by James et al (2019). In their analysis, they provide 4 beam efficiencies, the best and worst beam efficiency for closepack36 configuration and square6×6 configuration.…”

“…As for the beam efficiency of ASKAP, due to its fly'seye configuration, it's unacceptable to use a single airy disk to describe its beam efficiency. James et al (2019) derived the best and worst beam efficiencies for closep-ack36 and square6×6 configurations. Because the contribution from closepack36 configuration is much larger than that of square6×6, we take the best beam efficiency from closep-ack36 in our analysis.…”

A VOEvent-based automatic trigger system for the Murchison Widefield Array

“…In the absence of such an automated triggering service, Sokolowski et al (2018) have made use of shadowing observations to have the MWA co-observe with the Australian SKA Pathfinder (ASKAP, Johnston et al 2008;Hotan et al 2014). In this shadowing setup, ASKAP observes the sky in a fly's-eye mode whilst recording baseband data (James et al 2019b), which is then processed off-line to search for FRBs, resulting in 20 new detections (Bannister et al 2017;Shannon et al 2018). Simultaneously, the MWA observed an overlapping area of sky using the standard correlator mode (10 kHz / 0.5 s resolution).…”