A Shell Collision Scenario for EP251222b/GRB 251222A: Connecting Prompt Emission and Double-Peaked Afterglow

Run Shi; Di Xiao; Brajesh Kumar; Yifang Liang; Guowang Du; Jinjun Geng; Wenlong Zhang; Yuan Fang; Chenlei Guo; Dingfang Hu; Jie Hu; Qiuju Huang; Yanlong Hua; Yongfeng Huang; HongGuang Li; Xiangkun Liu; Xiaowei Liu; Yuan Liu; Jirong Mao; Xuan Mao; Bao Wang; Yuanpei Yang; Shiyan Zhong; Dan Zhu; Xuefeng Wu; Weimin Yuan

arXiv · 2610.21583

A Shell Collision Scenario for EP251222b/GRB 251222A: Connecting Prompt Emission and Double-Peaked Afterglow

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Abstract

GRB 251222A is a long-duration gamma-ray burst at a spectroscopic redshift of $z=3.171$. It was detected by the EP as the soft X-ray transient EP251222b before and during the gamma-ray emission. We performed a multi-wavelength (EP/WXT, fermi/GBM, swift/XRT and optical/NIR) investigation of this event. Its prompt soft X-ray and gamma-ray features, together with two afterglow rebrightenings, allow us to examine a possible connection between the prompt ejecta and the afterglow. We consider a multiple-shell scenario in which a faster, narrower shell catches up with a slower, wider shell. We assume that the selected pulse groups trace different ejecta and that the late X-ray flare marks their collision. Under these assumptions, we represent the post-collision outflow by an effective merged core and an outer annular component. A joint fit to the multi-band afterglow gives a Lorentz factor of $Γ_c\approx293$ for the merged core and $Γ_{1,w}\approx228$ for the outer component. The sample-wise collision mapping gives an energy release of $E_{\rm diss}=(1.6^{+2.1}_{-0.9})\times10^{52}$ erg and a dynamical dissipation efficiency of $0.37^{+0.07}_{-0.14}\%$. EP251222b can therefore be used to test a prompt--afterglow connection through shell collisions, although it does not uniquely establish this interpretation.

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Run Shi, Di Xiao, Brajesh Kumar, Yifang Liang, Guowang Du, Jinjun Geng, Wenlong Zhang, Yuan Fang, Chenlei Guo, Dingfang Hu, Jie Hu, Qiuju Huang, Yanlong Hua, Yongfeng Huang, HongGuang Li, Xiangkun Liu, Xiaowei Liu, Yuan Liu, Jirong Mao, Xuan Mao, Bao Wang, Yuanpei Yang, Shiyan Zhong, Dan Zhu, Xuefeng Wu, Weimin Yuan. 2026-10-15. A Shell Collision Scenario for EP251222b/GRB 251222A: Connecting Prompt Emission and Double-Peaked Afterglow. https://arxiv.org/abs/2610.21583

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