Relation between the keV-MeV and TeV emission of GRB 221009A and its implications

Yan-Qiu Zhang, Hao-Xiang Lin, Shao-Lin Xiong, Zhuo Li, Ming-Yu Ge, Chen-Wei Wang, Shu-Xu Yi, Zhen Zhang, Shuang-Nan Zhang, Li-Ming Song, Chao Zheng, Wang-Chen Xue, Jia-Cong Liu, Wen-Jun Tan, Yue Wang, Wen-Long Zhang

Published 2024-04-04Version 1

Gamma-ray bursts (GRBs) are believed to launch relativistic jets, which generate prompt emission by their internal processes and drive external shocks into surrounding medium, accounting for the long-lasting afterglow emission. However, how the jet powers the external shock is an open question. The unprecedented observations of the keV-MeV emission with GECAM and the TeV emission with LHAASO of so far the brightest burst, GRB 221009A, offer a great opportunity to study the prompt-to-afterglow transition and the early dynamical evolution of the external shock. In this letter, we find that the cumulative light curve of keV-MeV emission could well fit the rising stage of the TeV light curve of GRB 221009A, with a time delay of $4.45^{+0.26}_{-0.26}$\,s for TeV emission. Moreover, both the rapid increase in the initial stage and the excess from about \T+260\,s to 270\,s in the TeV light curve could be interpreted by inverse Compton (IC) scatterings of the inner-coming photons by the energetic electrons in external shock. Our results not only reveal a close relation between the keV-MeV and TeV emission, but also indicate a continuous, rather than impulsive, energy injection to the external shock. Assuming an energy injection rate proportional to the keV-MeV flux, we build a continuous energy injection model which well fits the TeV light curve of GRB 221009A, and provides an estimate of the Lorentz factor of the jet.