TITLE: GCN CIRCULAR NUMBER: 36770 SUBJECT: EP240626a: EP-FXT detection of the X-ray emission DATE: 24/06/27 05:45:49 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

Q.Y. Wu, Z. Z. Lv (NAOC, CAS), Q. C. Shui (IHEP, CAS), Z. X. Ling, Y. Liu, 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, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, 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, J. Zhang, 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 EP240626a (Wu et al., GCN 36766), we performed an observation of EP240626a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The observation began at 2024-06-26T08:58:37 (UTC), about 2.5 hours after the EP-WXT trigger. The exposure time is 6266 seconds. A faint source was detected at R.A. = 263.0171 deg, DEC = -13.0490 deg, with an uncertainty of 30 arcsec (radius, 90% C.L. statistical and systematic). The spectrum can be fitted with an absorbed power-law model with a photon index of 1.2(+0.7/-0.6) and the column density fixed at the Galactic value of 1.8 x 10^21 cm^-2. The derived unabsorbed flux in 0.5-10 keV is 1.5(+1.2/-0.7) 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 EP240626a.

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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