Spectral Hardening Reveals Afterglow Emergence in Long-duration Fast X-Ray Transients: A Case Study of GRB 250404A/EP250404a by Yi-Han Iris Yin, Yuan Fang, Bin-Bin Zhang, Chen Deng, Jun Yang, Run-Chao Chen, Yuan Liu, Yehao Cheng, Dong Xu, Xiaofeng Wang, Rongfeng Shen, Rui-Zhi Li, Jirong Mao, Wen-Xiong Li, Alberto Javier Castro-Tirado, Weihua Lei, Shao-Yu Fu, Yuan-Pei Yang, Shuai-Qing Jiang, Jie An, Chun Chen, Zhong-Nan Dong, Guowang Du, Ali Esamdin, Zhou Fan, Haicheng Feng, Lu Feng, Emilio Fernández-García, Xing Gao, Maria Gritsevich, Wei-Jian Guo, Jingwei Hu, You-Dong Hu, Yanlong Hua, Abdusamatjan Iskandar, Junjie Jin, Niu Li, Xia Li, Ziwei Li, Jia-Qi Lin, Dezi Liu, Jinzhong Liu, Qichun Liu, Xiaowei Liu, Xing Liu, Daniele B. Malesani, Ignacio Pérez-García, Hui Sun, Xue-Feng Wu, Yun-Ao Xiao, Ding-Rong Xiong, Shengyu Yan, Beibei Zhang, Jinghua Zhang, Haichang Zhu, Zipei Zhu, Hu Zou, Weimin Yuan, Bing Zhang

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