Einstein Probe discovery of EP240408a:A peculiar X-ray transient with an intermediate timescale  

作　　者：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 P.U.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 Zhen Zhan 

机构地区：[1]National Astronomical Observatories,Chinese Academy of Sciences,Beijing 100101,China [2]School of Astronomy and Space Sciences,University of Chinese Academy of Sciences,Beijing 100049,China [3]Key Laboratory of Particle Astrophysics,Institute of High Energy Physics,Chinese Academy of Sciences,Beijing 100049,China [4]Institute of Space Sciences(ICE),CSIC,Barcelona E-08193,Spain [5]Institut d’Estudis Espacials de Catalunya(IEEC),Barcelona E-08034,Spain [6]Max Planck Institute for Extraterrestrial Physics,Garching 85748,Germany [7]Innovation Academy for Microsatellites,Chinese Academy of Sciences,Shanghai 201210,China [8]Department of Physics,University of Hong Kong,Hong Kong 999077,China [9]School of Physics and Astronomy,University of Leicester,Leicester LE17RH,UK [10]School of Physics and Astronomy,Sun Yat-sen University,Zhuhai 519082,China [11]Department of Physics,Anhui Normal University,Wuhu 241002,China [12]Shanghai Institute of Technical Physics,Chinese Academy of Sciences,Shanghai 200083,China [13]Physics Department,Tsinghua University,Beijing 100084,China [14]Nevada Center for Astrophysics,University of Nevada Las Vegas,Las Vegas NV 89154,USA [15]Department of Physics and Astronomy,University of Nevada Las Vegas,Las Vegas NV 89154,USA [16]South African Astronomical Observatory,Cape Town 7935,South Africa [17]Department of Astronomy,University of Cape Town,Rondebosch 7701,South Africa [18]Department of Physics,University of the Free State,Bloemfontein 9300,South Africa [19]Universit´e Paris Cit´e,CNRS,Astroparticule et Cosmologie,Paris F-75013,France [20]Universit´e Paris Cit´e,CEA Paris-Saclay,IRFU/DAp-AIM,Gif-sur-Yvette 91191,France [21]Department of Astronomy,Guangzhou University,Guangzhou 510006,China [22]Cosmic Dawn Center(DAWN),Copenhagen 2200,Denmark [23]Niels Bohr Institute,University of Copenhagen,Copenhagen 2200,Denmark [24]Department of Physics and Astronomy,Rutgers,the State University of New Jersey,Piscataway NJ 08854-8019,USA [25]Institute for Cosmic Ray Research,The University of Tok

出　　处：《Science China(Physics,Mechanics & Astronomy)》2025年第1期218-236,共19页中国科学:物理学、力学、天文学(英文版)

基　　金：based on data obtained with Einstein Probe,a space mission supported by Strategic Priority Program on Space Science of Chinese Academy of Sciences,in collaboration with ESA,MPE and CNES(Grant No.XDA15310000);the Strategic Priority Research Program of the Chinese Academy of Sciences(Grant No.XDB0550200);the National Key R&D Program of China(Grant No.2022YFF0711500);the support by the National Natural Science Foundation of China(Grant Nos.12333004,12321003,12103065,12373040,12021003,12025303,12393814,and 12203071);the China Manned Space Project(Grant Nos.CMS-CSST-2021-A13,and CMS-CSST-2021-B11);the Youth Innovation Promotion Association of the Chinese Academy of Sciences;supported by a Ramón y Cajal fellowship(Grant No.RYC2021-030888-I);financial support from AGAUR,CSIC,MCIN and AEI 10.13039/501100011033(Grant Nos.PID2023-151307NB-I00,PIE 20215AT016,CEX2020-001058-M,and 2021-SGR-01270)。

摘　　要：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 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 a power-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.

关 键 词：X-RAY transients Einstein Probe 

分 类 号：P172.2[天文地球—天文学] O434.1[机械工程—光学工程]