Spectral Hardening Reveals Afterglow Emergence in Long-duration Fast X-Ray Transients: A Case Study of GRB 250404A/EP250404a
Yi-Han Iris Yin, Y Fang, Bin‐Bin Zhang, Chen Deng, Jun Yang, Run-Chao Chen, Yuan Liu, Yehao Cheng, Dong Xu, Xiaofeng Wang, Rong-Feng Shen, R.L Li, Jirong Mao, Wenxiong Li, A. J. Castro‐Tirado, Wei‐Hua Lei, Shao-Yu Fu, Yuan-Pei Yang, Shuai-Qing Jiang, Jie An, Chun Chen, Z. X. Dong, Guowang Du, Ali Esamdin, Zhou Fan, Hai-Cheng Feng, Feng Lu, E. Fernández-García, Xing Gao, Maria Gritsevich, Wei-Jian Guo, J. Y. Hu, Y. D. Hu, Yan-Qiao Hua, Abdusamatjan Iskandar, Junjie Jin, Niu Li, Xia Li, Ziwei Li, Jiaqi 家琪 Lin 林, Dezi Liu, Jinzhong Liu, Qichun Liu, Xiaowei Liu, Xing Liu, D. Malesani, Ignacio Pérez-García, Hui Sun, Xue-Feng Wu, Yun-Ao Xiao, Dingrong Xiong, Shengyu Yan, Beibei Zhang, Jinghua Zhang, Haichang Zhu, Zhichao Zhu, Hu Zou, Weimin Yuan, Bing Zhang
Abstract
Abstract 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 X with an initial softening, followed by an evident hardening, eventually reaching a plateau at the value of ∼–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 ∼300 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.