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 2021 September 25-28 in a series of four papers.In this second paper of the series,we study the energy distribution of 881 bursts(defined as significant signals separated by dips down to the noise level)detected in the first four days of our 19 hr observational campaign spanning 17 days.The event rate initially increased exponentially but the source activity stopped within 24 hr 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±0.01 and-4.27±0.23,respectively.Assuming a radio radiative efficiency of ηr=10-4,the total isotropic energy of the bursts released during the four days when the source was active is already 3.9 x 1046 erg,exceeding～23％of the available magnetar dipolar magnetic energy.This challenges the magnetar models which invoke an inefficient radio emission(e.g.,synchrotron maser models).

National Astronomical Observatories,Chinese Academy of Sciences,Beijing 100101,China;University of Chinese Academy of Sciences,Beijing 100049,China;Research Center for Intelligent Computing Platforms,Zhejiang Laboratory,Hangzhou 311100,China;Department of Physics and Astronomy,University of Nevada,Las Vegas,NV 89154,USA;Nevada Center for Astrophysics,University of Nevada,Las Vegas,NV 89154,USA;NAOC-UKZN Computational Astrophysics Centre,University of KwaZulu-Natal,Durban 4000,South Africa;CSIRO Space and Astronomy,PO Box 76,Epping,NSW 1710,Australia;Xinjiang Astronomical Observatory,Chinese Academy of Sciences,Urumqi 830011,China;Kavli Institute for Astronomy and Astrophysics,Peking University,Beijing 100871,China;Department of Astronomy,Peking University,Beijing 100871,China

[1]Yong-Kun Zhang;Pei Wang;Yi Feng;Bing Zhang;Di Li;Chao-Wei Tsai;Chen-Hui Niu;Rui Luo;Ju-Mei Yao;Wei-Wei Zhu;Jin-Lin Han;Ke-Jia Lee;De-Jiang Zhou;Jia-Rui Niu;Jin-Chen Jiang;Wei-Yang Wang;Chun-Feng Zhang;Heng Xu;Bo-Jun Wang;Jiang-Wei Xu-.FAST Observations of an Extremely Active Episode of FRB 20201124A.Ⅱ.Energy Distribution)[J].天文和天体物理学研究,2022(12):70-80

20201124A,magnetar,invoke,maser

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