The slope of the source-count distribution of fast radio burst (FRB) fluences, $alpha$, has been estimated using a variety of methods. Hampering all attempts have been the low number of detected FRBs, and the difficulty of defining a completeness threshold for FRB surveys. In this work, we extend maximum-likelihood methods for estimating $alpha$, using detected and threshold signal-to-noise ratios applied to all FRBs in a sample without regard to a completeness threshold. Using this method with FRBs detected by the Parkes radio telescope, we find $alpha=-1.18 pm 0.24$ (68% confidence interval, C.I.), i.e. consistent with a non-evolving Euclidean distribution ($alpha=-1.5$). Applying these methods to the Australian Square Kilometre Array Pathfinder (ASKAP) Commensal Real-time ASKAP Fast Transients (CRAFT) FRB survey finds $alpha=-2.2 pm 0.47$ (68% C.I.). A full maximum-likelihood estimate finds an inconsistency with the Parkes rate with a p-value of 0.86% ($2.6, sigma$). If not due to statistical fluctuations or biases in Parkes data, this is the first evidence for deviations from a pure power law in the integral source-count distribution of FRBs. It is consistent with a steepening of the integral source-count distribution in the fluence range 5--40,Jy,ms, for instance due to a cosmological population of FRB progenitors evolving more rapidly than the star-formation rate, and peaking in the redshift range 1--3.
Download