Implications from ASKAP Fast Radio Burst Statistics

Abstract

Although there has recently been tremendous progress in studies of fast radio bursts (FRBs), the nature of their progenitors remains a mystery. We study the fluence and dispersion measure (DM) distributions of the ASKAP sample to better understand their energetics and statistics. We first consider a simplified model of a power-law volumetric rate per unit isotropic energy dN/dE ∝ E^(−γ) with a maximum energy E_(max) in a uniform Euclidean universe. This provides analytic insights for what can be learned from these distributions. We find that the observed cumulative DM distribution scales as N(>DM) ∝ DM^(5−2γ) (for γ > 1) until a maximum DMmax above which bursts near E_(max) fall below the fluence threshold of a given telescope. Comparing this model with the observed fluence and DM distributions, we find a reasonable fit for γ ~ 1.7 and E_(max) ~ 10^(33) erg Hz^(−1). We then carry out a full Bayesian analysis based on a Schechter rate function with cosmological factors. We find roughly consistent results with our analytical approach, although with large errors on the inferred parameters due to the small sample size. The power-law index and the maximum energy are constrained to be γ ≃ 1.6 ± 0.3 and log(E_(max)) [erg/Hz] ≃ 34.1^(+1.1)_(-0.7) (68% confidence), respectively. From the survey exposure time, we further infer a cumulative local volumetric rate of log N(E > 10^(32) erg/Hz) [Gpc^(-3) yr^(-1)] ≃ 2.6 ± 0.4 (68% confidence). The methods presented here will be useful for the much larger FRB samples expected in the near future to study their distributions, energetics, and rates.