The slope of the source-count distribution of fast radio burst (FRB) fluences, α, 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 α, 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 α=−1.18±0.24 (68\% confidence interval, C.I.), i.e.\ consistent with a non-evolving Euclidean distribution (α=−1.5). Applying these methods to the Australian Square Kilometre Array Pathfinder (ASKAP) Commensal Real-time ASKAP Fast Transients (CRAFT) FRB survey finds α=−2.2±0.47 (68\% C.I.). A full maximum-likelihood estimate finds an inconsistency with the Parkes rate with a p-value of 0.86\% (2.6σ). 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.
@article{arxiv.1810.04357,
title = {The Slope of the Source-Count Distribution for Fast Radio Bursts},
author = {C. W. James and R. D. Ekers and J. -P. Macquart and K. W. Bannister and R. M. Shannon},
journal= {arXiv preprint arXiv:1810.04357},
year = {2018}
}