Probing the maximum energy of fast radio bursts using thousands of sources from the Second CHIME/FRB Catalog

Abstract

Quantifying the maximum energy of fast radio bursts (FRBs) can provide stringent constraints on their emission mechanisms and progenitor models. However, the most energetic bursts are rare, requiring a large sample of FRBs to detect them. In this work, we use the largest available such sample, 2,998 one-off FRBs from the Second CHIME/FRB Catalog, to obtain a lower limit on the maximum energy ($E^{\mathrm{max}}_{\mathrm{iso}}$) of FRBs, assuming isotropic energy distribution from FRB sources. In the absence of known redshifts ($z$) for most sources, we present a framework that uses the dispersion measures (DMs) and fluences of these FRBs, together with the probability distribution of $z$ given DM, to derive the lower limit on $E^{\mathrm{max}}_{\mathrm{iso}}$. We generate simulated FRB samples assuming different parameter values for a log-normal $\mathrm{DM}_{\mathrm{host}}$ distribution and a Schechter function form of the FRB energy function to estimate how many outliers -- FRBs with large DM contributions from the host galaxy or intervening galaxy halos -- could artificially inflate this limit. After accounting for outliers, the lower limit on $E^{\mathrm{max}}_{\mathrm{iso}}$ from Catalog 2 FRBs ranges between $1.2\times10^{41}$ and $1.9\times10^{42}$ erg, with best estimate $1.2\times10^{42}$ erg. This limit is consistent with those derived from much smaller FRB samples. Moreover, inferred energies of hundreds of FRBs appear collectively limited around $\sim10^{42}$ erg, suggesting a physical limit on the energy reservoir of FRB sources. The corresponding isotropic-equivalent FRB source energy is consistent with the total energy available in a magnetar's external dipole magnetic field, supporting magnetars as FRB progenitors.

Figures (1)

• Figure 1: For the simulated FRB sample having the best-fit values of $\mu_{\mathrm{host}}$, $\sigma_{\mathrm{host}}$, $E^{\mathrm{cut-off}}_{\mathrm{iso}}$, and $\gamma$ from Shin_FRB_pop, Panel \ref{['fig:Emaxll']} shows $P(E_{\text{iso}} > E_{\text{trial}})$ (gray line) before removing the outliers. The lower limit on $E^{\mathrm{max}}_{\mathrm{iso}}$ (blue dashed line) is slightly higher than the true $E^{\mathrm{max}}_{\mathrm{iso}}$ (black dashed line). Panel \ref{['fig:outliers']} shows the median inferred energies with 68% confidence interval from $P(E_\text{iso} \mid \mathrm{DM})$ for the simulated FRBs plotted against their true energies. The black line shows where the inferred and true energies are equal. The highest inferred FRBs which are considered outliers are marked in red. Panel \ref{['fig:Emaxll_out']} shows $P(E_{\text{iso}} > E_{\text{trial}})$ (gray line) after removing the outliers. The lower limit on $E^{\mathrm{max}}_{\mathrm{iso}}$ (blue dashed line) in this case is consistent and overlaps with the true $E^{\mathrm{max}}_{\mathrm{iso}}$ (black dashed line). This describes our method of obtaining a realistic value for the lower limit on $E^{\mathrm{max}}_{\mathrm{iso}}$ for a simulated sample after removing a sub-sample of highest inferred energy bursts as outliers.