Modelling the energy distribution in CHIME/FRB Catalog-1

TL;DR

This study analyzes the intrinsic properties of FRBs from CHIME/FRB Catalog-1, revealing energy distribution patterns and potential multiple populations based on spectral index and energy, with implications for understanding FRB origins.

Contribution

It introduces a novel method of characterizing FRB energy distributions using energy tracks and identifies evidence for two distinct FRB populations based on energy ranges.

Findings

Energy scale shifts to higher values with increasing spectral index.

Two distinct energy ranges suggest possible two FRB populations.

High-energy distribution fits a modified Schechter function.

Abstract

We characterize the intrinsic properties of any FRB using its redshift $z$, spectral index $\alpha$ and energy $E_{33}$ in units of $10^{33} \, {\rm J}$ emitted across $2128 - 2848\; {\rm MHz}$ in the FRB's rest frame. Provided that $z$ is inferred from the measured extra-galactic dispersion measure $DM_{\rm Ex}$, the fluence $F$ of the observed event defines a track in $(\alpha, E_{33})$ space which we refer to as the "energy track". Here we consider the energy tracks for a sample of $254$ non-repeating low dispersion measure FRBs from the CHIME/FRB Catalog-1, and use these to determine $n(E_{33} \mid \alpha)$ the conditional energy distribution i.e. the number of FRBs in the interval $\Delta E_{33}$ given a value of $\alpha$. Considering $-10 \le \alpha \le 10$, we find that the entire energy scale shifts to higher energy values as $\alpha$ is increased. For all values of $\alpha$, we can identify two distinct energy ranges indicating that there are possibly two distinct FRB populations. At high energies, the distribution is well fitted by a modified Schechter function whose slope and characteristic energy both increase with $\alpha$. At low energies, the number of FRBs are in excess of the predictions of the modified Schechter function indicating that we may have a distinctly different population of low-energy FRBs. We have checked that our main findings are reasonably robust to the assumptions regarding the Galactic Halo and Host galaxy contributions to the dispersion measure.