Einstein Probe discovery of EP240408a: a peculiar X-ray transient with an intermediate timescale
Creators: name: Zhang, Wenda
name: Yuan, Weimin
name: Ling, Zhixing
name: Chen, Yong
name: Rea, Nanda
name: Rau, Arne
name: Cai, Zhiming
name: Cheng, Huaqing
name: Zelati, Francesco Coti
name: Dai, Lixin
name: Hu, Jingwei
name: Jia, Shumei
name: Jin, Chichuan
name: Li, Dongyue
name: O'Brien, Paul
name: Shen, Rongfeng
name: Shu, Xinwen
name: Sun, Shengli
name: Sun, Xiaojin
name: Wang, Xiaofeng
name: Yang, Lei
name: Zhang, Bing
name: Zhang, Chen
name: Zhang, Shuang-Nan
name: Zhang, Yonghe
name: An, Jie
name: Buckley, David
name: Coleiro, Alexis
name: Cordier, Bertrand
name: Dou, Liming
name: Eyles-Ferris, Rob
name: Fan, Zhou
name: Feng, Hua
name: Fu, Shaoyu
name: Fynbo, Johan P. U.
name: Galbany, Lluis
name: Jha, Saurabh W.
name: Jiang, Shuaiqing
name: Kong, Albert
name: Kuulkers, Erik
name: Lei, Weihua
name: Li, Wenxiong
name: Liu, Bifang
name: Liu, Mingjun
name: Liu, Xing
name: Liu, Yuan
name: Liu, Zhu
name: Maitra, Chandreyee
name: Marino, Alessio
name: Monageng, Itumeleng
name: Nandra, Kirpal
name: Sanders, Jeremy
name: Soria, Roberto
name: Tao, Lian
name: Wang, Junfeng
name: Wang, Song
name: Wang, Tinggui
name: Wang, Zhongxiang
name: Wu, Qingwen
name: Wu, Xuefeng
name: Xu, Dong
name: Xu, Yanjun
name: Xue, Suijian
name: Xue, Yongquan
name: Zhang, Zijian
name: Zhu, Zipei
name: Zou, Hu
name: Bao, Congying
name: Chen, Fansheng
name: Chen, Houlei
name: Chen, Tianxiang
name: Chen, Wei
name: Chen, Yehai
name: Chen, Yifan
name: Cui, Chenzhou
name: Cui, Weiwei
name: Dai, Yanfeng
name: Fan, Dongwei
name: Guan, Ju
name: Han, Dawei
name: Hou, Dongjie
name: Hu, Haibo
name: Huang, Maohai
name: Huo, Jia
name: Jia, Zhenqing
name: Jiang, Bowen
name: Jin, Ge
name: Li, Chengkui
name: Li, Junfei
name: Li, Longhui
name: Li, Maoshun
name: Li, Wei
name: Li, Zhengda
name: Lian, Tianying
name: Liu, Congzhan
name: Liu, Heyang
name: Liu, Huaqiu
name: Lu, Fangjun
name: Luo, Laidan
name: Ma, Jia
name: Mao, Xuan
name: Pan, Haiwu
name: Pan, Xin
name: Song, Liming
name: Sun, Hui
name: Tan, Yunyin
name: Tang, Qingjun
name: Tao, Yihan
name: Wang, Hao
name: Wang, Juan
name: Wang, Lei
name: Wang, Wenxin
name: Wang, Yilong
name: Wang, Yusa
name: Wu, Qinyu
name: Xu, Haitao
name: Xu, Jingjing
name: Xu, Xinpeng
name: Xu, Yunfei
name: Xu, Zhao
name: Xue, Changbin
name: Xue, Yulong
name: Yan, Ailiang
name: Yang, Haonan
name: Yang, Xiongtao
name: Yang, Yanji
name: Zhang, Juan
name: Zhang, Mo
name: Zhang, Wenjie
name: Zhang, Zhen
name: Zhang, Zhen
name: Zhang, Ziliang
name: Zhao, Donghua
name: Zhao, Haisheng
name: Zhao, Xiaofan
name: Zhao, Zijian
name: Zhou, Hongyan
name: Zhou, Yilin
name: Zhu, Yuxuan
name: Zhu, Zhencai
Publisher: name: arXiv
Year: 2024
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: 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.
description: 25 pages, 11 figures