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 1500h 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^char_v = 2.09^+3.78_-1.04 × 10³² ergHz^-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 = 10¹⁵ G) would deplete the energy stored in its magnetosphere in ∼2150h, assuming a radio efficiency ϵ_radio = 10^-5. We find that the high-energy bursts (E_v > 3 × 10³⁰ ergHz^-1) play an important role in exhausting the energy budget of the source.