FAST Observations of an Extremely Active Episode of FRB 20201124A. II. Energy Distribution

Abstract We report the properties of more than 800 bursts detected from repeating fast radio burst (FRB) source FRB 20201124A with Five-hundred-meter Aperture Spherical Telescope (FAST) during an extremely active episode on UTC 2021 September 25–28 in a series four papers. In this second paper series, we study energy distribution 881 (defined as significant signals separated by dips down to noise level) first days our 19 hr observational campaign spanning 17 days. The event rate initially increased exponentially but activity stopped within 24 after 4th day. detection 542 one hour fourth day marked highest single so far. have complex structures time-frequency space. find double-peak waiting time, which can be modeled two log-normal functions peaking at 51.22 ms and 10.05 s, respectively. Compared emission previous FAST, peak time is smaller, suggesting that defined level source. calculate isotropic using both partial bandwidth full not significantly changed. connected broken-power law function fit cumulative well, lower higher-energy indices being −1.22 ± 0.01 −4.27 0.23, Assuming radiative efficiency η r = 10 −4 , total released when was already 3.9 × 46 erg, exceeding ∼23% available magnetar dipolar magnetic energy. This challenges models invoke inefficient (e.g., synchrotron maser models).