Synchrotron Radiation Dominates the Extremely Bright GRB 221009A
Jun Yang, Xiaohong Zhao, Zhen-Yu Yan, X. Wang, Yan-Qiu Zhang, Zhenghua An, Ce Cai, Xinqiao Li, Zihan Li, Jia-Cong Liu, Zi-Ke Liu, Xiang Ma, Yan-Zhi Meng, Wen-Xi Peng, Rui Qiao, Lang Shao, L. M. Song, Wenjun Tan, Ping Wang, Chen-Wei Wang, Xiang-Yang Wen, Shuo Xiao, W. C. Xue, Yu-Han Yang, Yi-Han Iris Yin, Bing Zhang, Fan Zhang, Shuai Zhang, Shuang‐Nan Zhang, Chao Zheng, S. J. Zheng, S. L. Xiong, Bin‐Bin Zhang
Abstract
Abstract The brightest gamma-ray burst, GRB 221009A, has spurred numerous theoretical investigations, with particular attention paid to the origins of ultrahigh-energy TeV photons during the prompt phase. However, analyzing the mechanism of radiation of photons in the ∼MeV range has been difficult because the high flux causes pileup and saturation effects in most GRB detectors. In this Letter, we present systematic modeling of the time-resolved spectra of the GRB using unsaturated data obtained from the Fermi Gamma-ray Burst Monitor (precursor) and SATech-01/GECAM-C (main emission and flare). Our approach incorporates the synchrotron radiation model, which assumes an expanding emission region with relativistic speed and a global magnetic field that decays with radius, and successfully fits such a model to the observational data. Our results indicate that the spectra of the burst are fully in accordance with a synchrotron origin from relativistic electrons accelerated at a large emission radius. The lack of thermal emission in the prompt emission spectra supports a Poynting flux–dominated jet composition.