2020ApJ...895L..22J

The discovery of many repeating fast radio bursts (FRBs) by the Canadian Hydrogen Intensity Mapping Experiment, the high rate of individual bursts, and the observation of repeat bursts from a bright FRB initially detected by the Australian Square Kilometre Array Pathfinder (ASKAP) suggest a significant population of repeating FRBs. Here, we analyze the rate distribution of this population using results from follow-up observations of the Commensal Real-time ASKAP Fast Transients Survey. Characterizing a repeating FRB by its burst rate R₀ above an energy of 10³⁸ erg, we consider a volumetric density Φ(R₀) with rate distribution dΦ∝R₀^ζ^dR₀. We use maximum-likelihood methods to constrain ζ < -1.94 at 90% confidence. We discuss how this constraint can be used to limit different classes of progenitor models for repeating FRBs: it excludes FRBs with burst rate proportional to neutron star spin-down power with braking index n <= 14, i.e., magnetic dipole radiation (n = 3); and it excludes some scenarios for FRB emission from magnetars, and the magnetic field interactions of compact binaries.