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| Format: | Preprint |
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2025
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| Online Access: | https://arxiv.org/abs/2506.00435 |
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| _version_ | 1866912545382596608 |
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| author | 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 |
| author_facet | 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 |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2506_00435 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Spectral Hardening Reveals Afterglow Emergence in Long-Duration Fast X-ray Transients: A Case Study of GRB 250404A/EP250404a 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 High Energy Astrophysical Phenomena 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. |
| title | Spectral Hardening Reveals Afterglow Emergence in Long-Duration Fast X-ray Transients: A Case Study of GRB 250404A/EP250404a |
| topic | High Energy Astrophysical Phenomena |
| url | https://arxiv.org/abs/2506.00435 |