A bimodal burst energy distribution of a repeating fast radio burst source

TL;DR

This study analyzes a large dataset of repeating fast radio bursts from FRB 121102, revealing a bimodal energy distribution, high burst rate, and lack of periodicity, which constrains models of their physical origin.

Contribution

It provides the first detailed characterization of the faint end of the energy distribution and demonstrates the bimodal nature of burst energies in a repeating FRB source.

Findings

Bimodal energy distribution characterized by a log-normal and a generalized Cauchy function.

Detected 1652 bursts with a peak rate of 122 per hour over 47 days.

No periodicity or quasi-periodicity found, challenging models with a single rotating object.

Abstract

The event rate, energy distribution, and time-domain behaviour of repeating fast radio bursts (FRBs) contains essential information regarding their physical nature and central engine, which are as yet unknown. As the first precisely-localized source, FRB 121102 has been extensively observed and shows non-Poisson clustering of bursts over time and a power-law energy distribution. However, the extent of the energy distribution towards the fainter end was not known. Here we report the detection of 1652 independent bursts with a peak burst rate of 122~hr^{-1}, in 59.5 hours spanning 47 days. A peak in the isotropic equivalent energy distribution is found to be ~4.8 x 10^{37} erg at 1.25~GHz, below which the detection of bursts is suppressed. The burst energy distribution is bimodal, and well characterized by a combination of a log-normal function and a generalized Cauchy function. The large number of bursts in hour-long spans allow sensitive periodicity searches between 1 ms and 1000 s. The non-detection of any periodicity or quasi-periodicity poses challenges for models involving a single rotating compact object. The high burst rate also implies that FRBs must be generated with a high radiative efficiency, disfavoring emission mechanisms with large energy requirements or contrived triggering conditions.