TITLE: GCN CIRCULAR NUMBER: 36760 SUBJECT: EP240625a: EP-FXT detection of the X-ray emission DATE: 24/06/25 16:02:06 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

Following the detection of the fast X-ray transient EP240625a (Pan et al., GCN 36757), we performed an observation of EP240625a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The observation began at 2024-06-25T03:10:00, about 1 hour after the EP-WXT detection. The exposure time is 5780 seconds. A slowly fading X-ray source was detected at R.A. = 310.7308 deg, DEC = -15.9760 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 3.85 x 10^20 cm^-2 and a photon index of 1.7(+/-0.4). The derived unabsorbed flux in 0.5-10 keV is 1.4(+0.8/-0.5) x 10^-13 erg/s/cm^2 (90% C.L.). There is no cataloged X-ray source within the error circle, suggesting this source being associated with EP240625a.

Please note that EP-FXT is currently undergoing in-flight calibration. The derived source parameters may be subject to larger uncertainties, so please use them with caution. 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).

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