Extragalactic fast X-ray transient from a weak relativistic jet associated with a Type Ic-BL supernova
Authors: Gao He , Sun Hui , Li Wenxiong , Liu Liangduan , Wang Xiaofeng , Zhang Bing , Yuan Weimin , Filippenko Alexei , Ahumada Tomas , Kasliwal Mansi , Stein Robert , Karambelkar Viraj , Cordier Bertrand , Wang Lingzhi , Fynbo Johan , DeLaunay James , Dai Z. G. , Lei Wei-Hua , Wang Chenyu , Zheng Weikang , Brink Thomas , Kuulkers Erik , Benetti Stefano , Elias-Rosa Nancy , Cai Z.M. , Chen Yehai , Liu Huaqiu , Zhang Y.-H , Zhou Yilin , Zhu Zhencai , Peng Zong-Kai , Wang Lei , Rea Nanda , Castro-Tirado Alberto , Guo Zhen , Ridnaia Anna , Svinkin Dmitry , Chen Y. , Cui Weiwei , Li C.K. , Zhang Shuang-Nan , Han D.W. , Wang J. , Li W. , Yang Y.J. , Wang Y.S. , Wang Hao , Ma Jia , Zhao X.F. , Lu Fangjun , Liu C.Z. , Yang Xiongtao , Luo Laidan , Xu Jingjing , Li M.S. , Huo J. , Chen Tianxiang , Hou Dongjie , Zhang Z.L. , Zhao Zijian , Zhu Y.X. , Jia S.M. , Song Liming , Zhao H.S. , Zhang J. , Guan J. , Feng Hua , Chen Houlei , Tang Qingjun , Andrews Jennifer , Terreran Giacomo , Howell D. Andrew , Farah Joseph , McCully Curtis , Newsome Megan , Gonzalez Estefania , Nandra Kirpal , Rau Arne , Sanders Jeremy , Fan Zhou , Guo Weijian , Jin Junjie , Xue Suijian , Zhu Zi-Pei , Wu Qinyu , An Jie , Fu Shao-Yu , Jiang Shuai-Qing , Ling Z.-X. , Liu Xing , Cheng H.-Q. , Fan Dongwei , Hu J.-W. , Liu Yuan , Liu H.-Y. , Pan Xin , Zhang C. , Hu Haibo , Li D.-Y. , Pan Haiwu , Tao Yihan , Zhang Mo , Zhang W.-D. , Zhang Wenjie , Lian Tianying , Chen Wei , Cui Chenzhou , Huang Maohai , Jin Chichuan , Liu M. , Mao Xuan , Wang Yilong , Xu Xinpeng , Xu Yunfei , Yang Haonan , Zhang Zhen , Xu Dong , Dai Yanfeng , Zhao D.-H. , Jia Zhenqing , Wang Wenxin , Tinyanont Samaporn , Tan Yunyin , Xu Haitao , Xue Changbin , Ai Shunke , Jiang Bowen , Jin Ge , Li Longhui , Xu Zhao , Zhang Zhen , Hu Maokai , Xiang Danfeng , Yang Yi , Wei Daming , Wu Xue-Feng , Liang Yifang , Dai Cuiyuan , Zhang Binbin , Huang Yong-Feng , Wang Xiang-Yu , Yang Jun , Li An , Li Jin-Da , Wen Xudong , O'Brien Paul , Liu Yuan-Qi , Wang Ailing , An Tao , Chen Fansheng , Chen Yifan , Li Junfei , Li Zhengda , Sun S.-L , Sun X.-J , Xue Yulong , Yan Ailiang , Mirzaqulov Davron , Ehgamberdiev Shuhrat , BAI Jinming , Feng Hai-Cheng , Li Rui-Zhi , Mao Jirong , Wang Baiting , Xiong Dingrong 
Published: November 2024

Massive stars end their life as core-collapse supernovae, amongst which some extremes are Type Ic broad-lined supernovae associated with long-duration gamma-ray bursts (LGRBs) having powerful relativistic jets. Their less-extreme brethren make unsuccessful jets that are choked inside the stars, appearing as X-ray flashes or low-luminosity GRBs. On the other hand, there exists a population of extragalactic fast X-ray transients (EFXTs) with timescales ranging from seconds to thousands of seconds, whose origins remain obscure. Known sources that contribute to the observed EFXT population include the softer analogs of LGRBs, shock breakouts of supernovae, or unsuccessful jets. Here, we report the discovery of the bright X-ray transient EP240414a detected by the Einstein Probe (EP), which is associated with the Type Ic supernova SN 2024gsa at a redshift of 0.401. The X-ray emission evolution is characterised by a very soft energy spectrum peaking at < 1.3 keV, which makes it distinct from known LGRBs, X-ray flashes, or low-luminosity GRBs. Follow-up observations at optical and radio bands revealed the existence of a weak relativistic jet that interacts with an extended shell surrounding the progenitor star. Located on the outskirts of a massive galaxy, this event reveals a new population of explosions of Wolf-Rayet stars characterised by a less powerful engine that drives a successful but weak jet, possibly owing to a progenitor star with a smaller core angular momentum than in traditional LGRB progenitors.