Spectral Hardening Reveals Afterglow Emergence in Long-duration Fast X-Ray Transients: A Case Study of GRB 250404A/EP250404a

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.

Authors

Institutions

Publication Details

Journal
The Astrophysical Journal Letters
Published
2025-08-14
DOI
https://doi.org/10.3847/2041-8213/adf552
Citations
8
Primary Topic
Gamma-ray bursts and supernovae
Type
article
Field-Weighted Citation Impact
9.24

Funders

Controls
|||
ALL TIME
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
article

Spectral Hardening Reveals Afterglow Emergence in Long-duration Fast X-Ray Transients: A Case Study of GRB 250404A/EP250404a

Xia Li, Yuan Liu, Maria Gritsevich, Yuan-Pei Yang et al.
8 citations
The Astrophysical Journal Letters
Gamma-ray bursts and supernovae
9.24
article

Spectral Hardening Reveals Afterglow Emergence in Long-duration Fast X-Ray Transients: A Case Study of GRB 250404A/EP250404a

Xia Li, Yuan Liu, Maria Gritsevich, Yuan-Pei Yang, Weimin Yuan, Run-Chao Chen, Shao-Yu Fu, Z. X. Dong, Y Fang, D. Malesani, Yi-Han Iris Yin, R.L Li, Hui Sun, Hong Zhu, Y. D. Hu, Junjie Jin, Dezi Liu, Shengyu Yan, Dingrong Xiong, Jinzhong Liu, Abdusamatjan Iskandar, Yan-Qiao Hua, Rong-Feng Shen, Chen Deng, A. J. Castro‐Tirado, Ali Esamdin, Ziwei Li, E. Fernández-García, Jie An, Wenxiong Li, Jun Yang, Xue-Feng Wu, J. Y. Hu, Guowang Du, Ignacio Pérez-García, Hai-Cheng Feng, Wei‐Hua Lei, Jiaqi 家琪 Lin 林, Yehao Cheng, Xiaofeng Wang, Dong Xu, Bing Zhang, Xiaowei Liu, Xing Liu, Zhou Fan, Niu Li, Bin‐Bin Zhang, Jinghua Zhang, Beibei Zhang, Wei-Jian Guo, Feng Lu, Jirong Mao, Qichun Liu, Yun-Ao Xiao, Hu Zou, Shuai-Qing Jiang, Zhichao Zhu, Xing Gao, Chun Chen
article en
8 citations

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.

The Astrophysical Journal LettersVol. 989(2)
University of Copenhagen (DK), University of Science and Technology of China (CN), Ural Federal University (RU), Ministry of Education of the People's Republic of China (CN), University of Helsinki (FI), University of Nevada, Las Vegas (US), Radboud University Nijmegen (NL), Guangxi University (CN), Sun Yat-sen University (CN), Yunnan University (CN), Chinese Academy of Sciences (CN), Beijing Normal University (CN), Xinjiang Astronomical Observatory (CN), Yunnan Observatories (CN), Purple Mountain Observatory (CN), Institute of Physics and Technology (RU), Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (CN), Altaiskiy State Nature Biosphere Reserve (RU), Astronomy and Space (AU), Instituto de Astrofísica de Andalucía (ES), National Astronomical Observatories (CN), University of Chinese Academy of Sciences (CN), Huazhong University of Science and Technology (CN), Universidad de Málaga (ES), Nanjing University (CN), University of Hong Kong (HK), Tsinghua University (CN)
European Commission, National Research Foundation, Danmarks Grundforskningsfond, National Natural Science Foundation of China, Chinese Academy of Sciences, Sun Yat-sen University, Centre National d’Etudes Spatiales, Yunnan University, Junta de Andalucía, Natural Science Foundation of Xinjiang, Agencia Estatal de Investigación, National Key Research and Development Program of China, Fundamental Research Funds for the Central Universities, National Astronomical Observatories, Chinese Academy of Sciences
Openalex Percentile: Top 2%
Gamma-ray bursts and supernovae
9.24
DOIOpen Access PDFOpenAlex
AI Navigator

Ask Laika to Summarize, Analyze, and Connect papers live on the map.

Summarize Papers & Methodologies

Extract key findings, datasets, and comparative methods across publications.

Benchmark Rankings & Visual Analytics

Rank top research institutions, authors, funders, topics, and journals by Field-Weighted Citation Impact (FWCI) and paper volume with instant charts.

Connect Distant Disciplines

Bridge topological clusters on the map to find hidden collaborative intersections.