FAST observations of an extremely active episode of FRB 20201124A: II. Energy Distribution

TL;DR

This study analyzes over 800 bursts from FRB 20201124A during an active episode, revealing detailed energy distribution, complex burst structures, and implications for magnetar models, with the highest event rate observed so far from a single FRB source.

Contribution

It provides the first detailed energy distribution analysis of a large burst sample from FRB 20201124A during an active episode, highlighting the burst energy spectrum and challenging existing magnetar emission models.

Findings

Burst energy distribution fits a broken power-law with indices -1.22 and -4.27.

Detected 542 bursts in one hour, the highest event rate from a single FRB source.

Total energy released exceeds 23% of the magnetar's magnetic energy, challenging certain models.

Abstract

We report the properties of more than 800 bursts detected from the repeating fast radio burst (FRB) source FRB 20201124A with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) during an extremely active episode on UTC September 25-28, 2021 in a series of four papers. In this second paper of the series, we mainly focus on the energy distribution of the detected bursts. The event rate initially increased exponentially but the source activity stopped within 24 hours after the 4th day. The detection of 542 bursts in one hour during the fourth day marked the highest event rate detected from one single FRB source so far. The bursts have complex structures in the time-frequency space. We find a double-peak distribution of the waiting time, which can be modeled with two log-normal functions peaking at 51.22 ms and 10.05 s, respectively. Compared with the emission from a previous active episode of the source detected with FAST, the second distribution peak time is smaller, suggesting that this peak is defined by the activity level of the source. We calculate the isotropic energy of the bursts using both a partial bandwidth and a full bandwidth and find that the energy distribution is not significantly changed. We find that an exponentially connected broken-power-law function can fit the cumulative burst energy distribution well, with the lower and higher-energy indices being $-1.22\pm0.01$ and $-4.27\pm0.23$, respectively. Assuming a radio radiative efficiency of $\eta_r = 10^{-4}$, the total isotropic energy of the bursts released during the four days when the source was active is already $3.9\times10^{46}$ erg, exceeding $\sim 23\%$ of the available magnetar dipolar magnetic energy. This challenges the magnetar models invoking an inefficient radio emission (e.g. synchrotron maser models).