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Autori principali: Yin, Yi-Han Iris, Fang, Yuan, Zhang, Bin-Bin, Deng, Chen, Yang, Jun, Chen, Run-Chao, Liu, Yuan, Cheng, Yehao, Xu, Dong, Wang, Xiaofeng, Shen, Rongfeng, Li, Rui-Zhi, Mao, Jirong, Li, Wen-Xiong, Castro-Tirado, Alberto Javier, Lei, Weihua, Fu, Shao-Yu, Yang, Yuan-Pei, Jiang, Shuai-Qing, An, Jie, Chen, Chun, Dong, Zhong-Nan, Du, Guowang, Esamdin, Ali, Fan, Zhou, Feng, Haicheng, Feng, Lu, Fernández-García, Emilio, Gao, Xing, Gritsevich, Maria, Guo, Wei-Jian, Hu, Jingwei, Hu, You-Dong, Hua, Yanlong, Iskandar, Abdusamatjan, Jin, Junjie, Li, Niu, Li, Xia, Li, Ziwei, Lin, Jia-Qi, Liu, Dezi, Liu, Jinzhong, Liu, Qichun, Liu, Xiaowei, Liu, Xing, Malesani, Daniele B., Pérez-García, Ignacio, Sun, Hui, Wu, Xue-Feng, Xiao, Yun-Ao, Xiong, Ding-Rong, Yan, Shengyu, Zhang, Beibei, Zhang, Jinghua, Zhu, Haichang, Zhu, Zipei, Zou, Hu, Yuan, Weimin, Zhang, Bing
Natura: Preprint
Pubblicazione: 2025
Soggetti:
High Energy Astrophysical Phenomena
Accesso online:https://arxiv.org/abs/2506.00435
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Sommario:
  • The prompt emission and afterglow phases of gamma-ray bursts (GRBs) have been extensively studied, yet the transition between these two phases remains inadequately characterized due to limited multiwavelength observational coverage. Among the recent growing samples of fast X-ray transients observed by Einstein Probe (EP), a subgroup of GRBs are captured with long-duration X-ray emission, potentially containing featured evolution from prompt emission to the afterglow phase. In this Letter, we present a detailed analysis of GRB 250404A/EP250404a, a bright fast X-ray transient detected simultaneously by EP and the Fermi Gamma-ray Burst Monitor in X-rays and gamma rays. Its continuous X-ray emission reveals a long-duration tail, accompanied by distinct spectral evolution manifested by the spectral index $α_{\rm X}$ with an initial softening, followed by an evident hardening, eventually reaching a plateau at the value of $\sim$ -2. Early optical and near-infrared observations enable broadband modeling with forward- and reverse-shock components, confirming that the X-ray hardening signals the emergence of the external-shock afterglow. From this spectral hardening we infer that the prompt phase in soft X-rays lasted $\sim300\;\mathrm{s}$, which is more than 3 times longer than the gamma-ray $T_{90}$. This well-tracked soft-hard-flat spectral pattern provides a clear indication of afterglow emergence from the fading prompt emission and offers a practical criterion for identifying a distinct population of GRBs among fast X-ray transients, even when the detection of the gamma-ray counterpart or obvious temporal break is absent.

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