TITLE: GCN CIRCULAR NUMBER: 36757 SUBJECT: EP240625a: EP-WXT detection of a fast X-ray transient DATE: 24/06/25 11:04:29 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

X. Pan, Z. Z. Lv (NAOC, CAS), Y. C. Fu (BNU), Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, W. Chen, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, T. Y. Lian, X. Mao, H. W. Pan, H. Sun, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, S. N. Zhang (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA) on behalf of the Einstein Probe team

We report on the detection of a fast X-ray transient, designated EP240625a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient triggered the WXT on-board processing unit at 2024-06-25T01:48:23 (UTC). The position of the source is R.A. = 310.760 deg, DEC = -15.966 deg (J2000) with an uncertainty of 2 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 300 seconds and has a peak unabsorbed flux of ~1.3 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.8(+/-0.3) (with the column density fixed at the Galactic value of 3.85 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 2.9(+0.7/-0.5) x 10^-10 erg/s/cm^2. The uncertainties are at the 1 sigma level for the above parameters. No previously known X-ray sources at a similar flux level are found within the 2 arcmin region around the source position.

Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP. Further follow-up observations are encouraged to identify the nature of this X-ray transient.

The above observation was made with the WXT instrument during the commissioning phase of EP. Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.

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