New Insights into the Criteria of Fast Radio Burst in the Light of FRB 20121102A

TL;DR

This paper revisits the classification of fast radio bursts (FRBs) by proposing a brightness temperature criterion to distinguish between two physical types, revealing different mechanisms and a tight relation for classical FRBs, based on recent large datasets.

Contribution

It introduces a brightness temperature-based method to classify FRBs into two subtypes, linking physical origin to observable properties and suggesting variability in the dividing line across sources.

Findings

Bimodal distribution of FRBs suggests two physical types.

A critical brightness temperature of ~10^33 K separates classical and atypical FRBs.

Classical FRBs show a tight width-fluence relation.

Abstract

The total event number of fast radio bursts (FRBs) is accumulating rapidly with the improvement of existing radio telescopes and the completion of new facilities. Especially, the Five-hundred-meter Aperture Spherical radio Telescope (FAST) Collaboration has just reported more than one thousand bursts in a short observing period of 47 days \citep{LiD2021}. The interesting bimodal distribution in their work motivates us to revisit the definition of FRBs. In this work, we ascribe the bimodal distribution to two physical kinds of radio bursts, which may have different radiation mechanisms. We propose to use brightness temperature to separate two subtypes. For FRB 20121102A, the critical brightness temperature is $T_{\rm B,cri}\simeq10^{33}\,\rm K$. Bursts with $T_{\rm B}\geq T_{\rm B,cri}$ are denoted as "classical" FRBs, and further we find a tight pulse width-fluence relation ($T\propto\mathcal{F_\nu}^{0.306}$) for them. On the contrary, the other bursts are considered as "atypical" bursts that may originate from a different physical process. We suggest that for each FRB event, a similar dividing line should exist but $T_{\rm B,cri}$ is not necessarily the same. Its exact value depends on FRB radiation mechanism and properties of the source.