Einstein Probe discovery of EP240408a: A peculiar X-ray transient with an intermediate timescale
journal contribution
posted on 2024-12-09, 15:55 authored by Wenda Zhang, Weimin Yuan, Zhixing Ling, Yong Chen, Nanda Rea, Arne Rau, Zhiming Cai, Huaqing Cheng, Francesco Coti Zelati, Lixin Dai, Jingwei Hu, Shumei Jia, Chichuan Jin, Dongyue Li, Paul O’Brien, Rongfeng Shen, Xinwen Shu, Shengli Sun, Xiaojin Sun, Xiaofeng Wang, Lei Yang, Bing Zhang, Chen Zhang, Shuang-Nan Zhang, Yonghe Zhang, Jie An, David Buckley, Alexis Coleiro, Bertrand Cordier, Liming Dou, Rob Eyles-Ferris, Zhou Fan, Hua Feng, Shaoyu Fu, Johan PU Fynbo, Lluis Galbany, Saurabh W Jha, Shuaiqing Jiang, Albert Kong, Erik Kuulkers, Weihua Lei, Wenxiong Li, Bifang Liu, Mingjun Liu, Xing Liu, Yuan Liu, Zhu Liu, Chandreyee Maitra, Alessio Marino, Itumeleng Monageng, Kirpal Nandra, Jeremy Sanders, Roberto Soria, Lian Tao, Junfeng Wang, Song Wang, Tinggui Wang, Zhongxiang Wang, Qingwen Wu, Xuefeng Wu, Dong Xu, Yanjun Xu, Suijian Xue, Yongquan Xue, Zijian Zhang, Zipei Zhu, Hu Zou, Congying Bao, Fansheng Chen, Houlei Chen, Tianxiang Chen, Wei Chen, Yehai Chen, Yifan Chen, Chenzhou Cui, Weiwei Cui, Yanfeng Dai, Dongwei Fan, Ju Guan, Dawei Han, Dongjie Hou, Haibo Hu, Maohai Huang, Jia Huo, Zhenqing Jia, Bowen Jiang, Ge Jin, Chengkui Li, Junfei Li, Longhui Li, Maoshun Li, Wei Li, Zhengda Li, Tianying Lian, Congzhan Liu, Heyang Liu, Huaqiu Liu, Fangjun Lu, Laidan Luo, Jia Ma, Xuan Mao, Haiwu Pan, Xin Pan, Liming Song, Hui Sun, Yunyin Tan, Qingjun Tang, Yihan Tao, Hao Wang, Juan Wang, Lei Wang, Wenxin Wang, Yilong Wang, Yusa Wang, Qinyu Wu, Haitao Xu, Jingjing Xu, Xinpeng Xu, Yunfei Xu, Zhao Xu, Changbin Xue, Yulong Xue, Ailiang Yan, Haonan Yang, Xiongtao Yang, Yanji Yang, Juan Zhang, Mo Zhang, Wenjie Zhang, Zhen Zhang, Ziliang Zhang, Donghua Zhao, Haisheng Zhao, Xiaofan Zhao, Zijian Zhao, Hongyan Zhou, Yilin Zhou, Yuxuan Zhu, Zhencai ZhuWe report the discovery of a peculiar X-ray transient, EP240408a, by Einstein Probe (EP) and follow-up studies made with EP, Swift, NICER, GROND, ATCA and other ground-based multiwavelength telescopes. The new transient was first detected with Wide-field X-ray Telescope (WXT) on board EP on April 8th, 2024, manifested in an intense yet brief X-ray flare lasting for 12 s. The flare reached a peak flux of 3.9 × 10−9 erg cm−2 s−1 in 0.5–4 keV, ∼300 times brighter than the underlying X-ray emission detected throughout the observation. Rapid and more precise follow-up observations by EP/FXT, Swift and NICER confirmed the finding of this new transient. Its X-ray spectrum is non-thermal in 0.5–10 keV, with apower-law photon index varying within 1.8–2.5. The X-ray light curve shows a plateau lasting for ∼4 d, followed by a steep decay till becoming undetectable ∼10 d after the initial detection. Based on its temporal property and constraints from previous EP observations, an unusual timescale in the range of 7–23 d is found for EP240408a, which is intermediate between the commonly found fast and long-term transients. No counterparts have been found in optical and near-infrared, with the earliest observation at 17 h after the initial X-ray detection, suggestive of intrinsically weak emission in these bands. We demonstrate that the remarkable properties of EP240408a are inconsistent with any of the transient types known so far, by comparison with, in particular, jetted tidal disruption events, gamma-ray bursts, X-ray binaries and fast blue optical transients. The nature of EP240408a thus remains an enigma. We suggest that EP240408a may represent a new type of transients with intermediate timescales of the order of ∼10 d. The detection and follow-ups of more of such objects are essential for revealing their origin.
History
Author affiliation
College of Science & Engineering Physics & AstronomyVersion
- AM (Accepted Manuscript)
