Observed trends in FRB population and bi-modality in the luminosity density distribution

Fast radio bursts (FRBs) are radio-transients of extragalactic origin lasting from fractions of a millisecond to hundreds of milliseconds. Their actual physical nature is yet to be ascertained and is still a topic of active research. In this paper, we have considered both non-CHIME and CHIME sources, and have subjected the available FRB data to various analyses. Since CHIME first catalogue provides only the lower bounds to the FRB flux density and fluence, we have devised a novel approach that utilises the ratio of the catalogued lower limits of the flux density \(S_{\nu _O}\) to the fluence \(F_{\nu _O}\) of individual FRB events to construct several parameters \(Xi, i=1,2,\ldots ,7\) but \( i \ne 5\) to investigate the presence of underlying trends in the CHIME FRB population. These parameters are also computed for the non-CHIME FRB events using the ratio of the measured flux density \(S_{\nu _O}\) to the fluence \(F_{\nu _O}\). One of these defined parameters (X7) involve the actual brightness temperature as well as energy density instead of the corresponding bounds, despite one’s ignorance of thess actual size of the FRB emission region. Our first robust conclusion is that the individual non-CHIME FRB events fall under two broad categories based on their peak luminosity densities. This has been explicitly demonstrated for peak luminosity densities calculated at two distinct frequencies – 300 and 450 MHz. Our second robust result is that the parameters Xis involving the ratio of the flux density to the fluence are almost the same for both CHIME and non-CHIME FRB populations, vindicating our use of these parameters that make use of \(S_{\nu _O}/ F_{\nu _O}\) and other measured quantities. This universality is also seen in the underlying patterns exhibited by the distributions of the computed parameters Xis for both CHIME and non-CHIME FRB population, suggesting thereby the existence of two categories even for the FRB events detected by CHIME. Assuming that FRBs are caused by magnetar glitches that lead to an abrupt change in the light cylinder radius, we have considered a simple physical model to address the issue of two categories based on the FRB luminosity density.