A probe of the maximum energetics of fast radio bursts through a prolific repeating source

Fast radio bursts (FRBs) are sufficiently energetic to be detectable from luminosity distances up to at least seven billion parsecs (redshift $z > 1$). Probing the maximum energies and luminosities of FRBs constrains their emission mechanism and cosmological population. Here, we investigate the maximum energetics of a highly active repeater, FRB 20220912A, using 1500 h of observations. We detect 130 high-energy bursts and find a break in the burst energy distribution, with a flattening of the power-law slope at higher energy – consistent with the behaviour of another highly active repeater, FRB 20201124A. There is a roughly equal split of integrated burst energy between the low- and high-energy regimes. Furthermore, we model the rate of the highest energy bursts and find a turnover at a characteristic spectral energy density of $E^{\rm {char}}_{\nu } = 2.09^{+3.78}_{-1.04}\times 10^{32}$ erg Hz$^{-1}$. This characteristic maximum energy agrees well with observations of apparently one-off FRBs, suggesting a common physical mechanism for their emission. The extreme burst energies push radiation and source models to their limit: at this burst rate a typical magnetar ($B = 10^{15}$ G) would deplete the energy stored in its magnetosphere in $\sim$2150 h, assuming a radio efficiency $\epsilon _\mathrm{radio} = 10^{-5}$. We find that the high-energy bursts ($E_\nu > 3 \times 10^{30}$ erg Hz$^{-1}$) play an important role in exhausting the energy budget of the source.