Einstein Probe discovery of EP240408a: a peculiar X-ray transient with an intermediate timescale
Abstract
We 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 multi-wavelength 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 seconds. The flare reached a peak flux of 3.9x10^(-9) erg/cm2/s in 0.5-4 keV, about 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 a power-law photon index varying within 1.8-2.5. The X-ray light curve shows a plateau lasting for about 4 days, followed by a steep decay till becoming undetectable about 10 days after the initial detection. Based on its temporal property and constraints from previous EP observations, an unusual timescale in the range of 7-23 days 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 hours 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 about 10 days. The detection and follow-ups of more of such objects are essential for revealing their origin.
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@article{arxiv.2410.21617,
title = {Einstein Probe discovery of EP240408a: a peculiar X-ray transient with an intermediate timescale},
author = {Wenda Zhang and Weimin Yuan and Zhixing Ling and Yong Chen and Nanda Rea and Arne Rau and Zhiming Cai and Huaqing Cheng and Francesco Coti Zelati and Lixin Dai and Jingwei Hu and Shumei Jia and Chichuan Jin and Dongyue Li and Paul O'Brien and Rongfeng Shen and Xinwen Shu and Shengli Sun and Xiaojin Sun and Xiaofeng Wang and Lei Yang and Bing Zhang and Chen Zhang and Shuang-Nan Zhang and Yonghe Zhang and Jie An and David Buckley and Alexis Coleiro and Bertrand Cordier and Liming Dou and Rob Eyles-Ferris and Zhou Fan and Hua Feng and Shaoyu Fu and Johan P. U. Fynbo and Lluis Galbany and Saurabh W. Jha and Shuaiqing Jiang and Albert Kong and Erik Kuulkers and Weihua Lei and Wenxiong Li and Bifang Liu and Mingjun Liu and Xing Liu and Yuan Liu and Zhu Liu and Chandreyee Maitra and Alessio Marino and Itumeleng Monageng and Kirpal Nandra and Jeremy Sanders and Roberto Soria and Lian Tao and Junfeng Wang and Song Wang and Tinggui Wang and Zhongxiang Wang and Qingwen Wu and Xuefeng Wu and Dong Xu and Yanjun Xu and Suijian Xue and Yongquan Xue and Zijian Zhang and Zipei Zhu and Hu Zou and Congying Bao and Fansheng Chen and Houlei Chen and Tianxiang Chen and Wei Chen and Yehai Chen and Yifan Chen and Chenzhou Cui and Weiwei Cui and Yanfeng Dai and Dongwei Fan and Ju Guan and Dawei Han and Dongjie Hou and Haibo Hu and Maohai Huang and Jia Huo and Zhenqing Jia and Bowen Jiang and Ge Jin and Chengkui Li and Junfei Li and Longhui Li and Maoshun Li and Wei Li and Zhengda Li and Tianying Lian and Congzhan Liu and Heyang Liu and Huaqiu Liu and Fangjun Lu and Laidan Luo and Jia Ma and Xuan Mao and Haiwu Pan and Xin Pan and Liming Song and Hui Sun and Yunyin Tan and Qingjun Tang and Yihan Tao and Hao Wang and Juan Wang and Lei Wang and Wenxin Wang and Yilong Wang and Yusa Wang and Qinyu Wu and Haitao Xu and Jingjing Xu and Xinpeng Xu and Yunfei Xu and Zhao Xu and Changbin Xue and Yulong Xue and Ailiang Yan and Haonan Yang and Xiongtao Yang and Yanji Yang and Juan Zhang and Mo Zhang and Wenjie Zhang and Zhen Zhang and Zhen Zhang and Ziliang Zhang and Donghua Zhao and Haisheng Zhao and Xiaofan Zhao and Zijian Zhao and Hongyan Zhou and Yilin Zhou and Yuxuan Zhu and Zhencai Zhu},
journal= {arXiv preprint arXiv:2410.21617},
year = {2024}
}
Comments
25 pages, 11 figures