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

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 (Li et al. 2021). 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 B,cri ≃ 10 33 K. Bursts with T B ≥ T B,cri are denoted as "classical" FRBs, and further we find a tight pulse width-fluence relation (T ∝ F ν 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 B,cri is not necessarily the same. Its exact value depends on FRB radiation mechanism and properties of the source.

A Possible Subclassification of Fast Radio Bursts

“…But it is reasonable that the dividing lines of L ν and T B (and νW ) might be quite different from the ones of type I FRBs (namely L ν, crit = 10 34 erg/s/Hz and T B, crit = 2 × 10 35 K). For instance, a much lower T B, crit = 10 33 K was proposed for FRB 20121102A in [31], although their main purpose is somewhat different from ours. Here, we would like to make a bold but not too bold speculation.…”

“…, where y k , ŷk and ȳ are the observed value, regressed value and mean of observed value [31,76], respectively. 16) for 65 type Ia FRBs (blue points), but no such correlation for 365 type Ib FRBs (red points).…”

“…In e.g. [31], it was argued that the brightness temperature T B might be used to classify repeating bursts into classical (T B ≥ 10 33 K) and atypical (T B < 10 33 K) ones in the light of the well-known repeating FRB 20121102A. A tight powerlaw correlation between pulse width and fluence was found for classical bursts.…”

Fast Radio Bursts

“…They cluster in different regions on the spectral luminosity-duration phase space plot, and the main difference between them is brightness temperature [113]. The large FAST sample of FRB 20121102A has been classified in this way and even a two-parameter correlation for the classified bursts was found [172]. This has been further confirmed by a classification of FRBs in CHIME catalog [19].…”

“…Alternatively, it is worthwhile to look for more physical criteria of classification. One feasible candidate is brightness temperature [172]. Besides FRBs, there are many kinds of short-duration radio transients such as pulsar radio emission, giant pulses and nanoshots.…”