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 1,500 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 $E^{\textrm{char}}ν = 2.09^{+3.78}{-1.04}\times10^{32}$ erg/Hz. 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 $ε_\mathrm{radio} = 10^{-5}$. We find that the high-energy bursts ($E_ν> 3 \times 10^{30}$ erg Hz$^{-1}$) play an important role in exhausting the energy budget of the source.

💡 Research Summary

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This paper presents a comprehensive study of the maximum energetics of the highly active repeating fast radio burst (FRB) source FRB 20220912A. Using an unprecedented 1 500 hours of on‑source observing time across four European 25–32 m radio telescopes (Westerbork RT‑1, Onsala 25 m, Stockert 25 m, and Toruń 32 m), the authors collected data simultaneously at three frequency bands: P‑band (330 MHz), L‑band (1.4 GHz), and C‑band (4.7 GHz). The total campaign spanned 117 days between 2022 October 15 and 2023 February 8, yielding 2 192 hours of raw telescope time, which reduced to 1 491 hours of unique on‑source exposure after accounting for overlap.

Data were recorded either as raw voltage streams (VDIF format) or as total‑intensity (PFFTS) files, then converted to filterbank files using standard tools (digifil, SIGPROC, PRESTO). Burst searching employed Heimdall (for the voltage‑based data) and PRESTO’s single_pulse_search (for the total‑intensity data), with a signal‑to‑noise threshold of 7–8σ and a dispersion‑measure window of ±50 pc cm⁻³ around the known DM of 220 pc cm⁻³. Radio‑frequency interference was mitigated through static masks and the machine‑learning classifier FETCH; candidates with ≥50 % probability were manually inspected. The detection and completeness fluence thresholds were calculated via the radiometer equation, assuming typical burst widths of 1 ms (detection) and 3 ms (completeness).

The campaign yielded 130 high‑energy bursts: 114 unique events at L‑band (including 16 detected simultaneously by multiple telescopes) and 16 at P‑band; no bursts were seen at C‑band. Each burst was labeled Bx‑T (e.g., B15‑Tr) and its dynamic spectrum was examined. Fluences were measured using three independent digitisation pipelines (VDIF→digifil, SFXC, and digifil alone) to quantify systematic differences; the resulting fluences differed by ~30 % on average, reflecting digitisation uncertainties.

The authors constructed the cumulative burst‑rate distribution (R(>E_{\nu})) as a function of spectral energy density (E_{\nu}). Below (E_{\nu}\approx3\times10^{30}) erg Hz⁻¹ the distribution follows a power‑law with index (\gamma_D\simeq-1.5), consistent with previous population studies of apparently non‑repeating FRBs. Above this threshold the slope flattens to (\gamma_D\approx-0.8), indicating a relative excess of high‑energy events compared with a simple extrapolation. This “break” mirrors the behaviour reported for another hyper‑active repeater, FRB 20201124A, suggesting a common physical limitation in repeaters.

Fitting the high‑energy tail with a truncated power‑law yields a characteristic spectral energy density \