TITLE: GCN CIRCULAR NUMBER: 35951 SUBJECT: X-ray transient EP240315a: EP-FXT detection of the X-ray afterglow DATE: 24/03/18 13:38:26 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Y. Chen, S.M. Jia, W.W. Cui, C.K. Li, D.W. Han, J. Wang, W. Li, X.F. Zhao, J.J. Xu, H.S. Zhao, J. Guan, J. Zhang, L.M. Song, F.J. Lu, C.Z. Liu, S.N. Zhang (IHEP, CAS), W.J. Zhang, X. Mao, W.D. Zhang, D.Y. Li, T.Y. Lian, Y. Liu, W. Yuan (NAOC, CAS), K. Nandra, A. Rau, P. Friedrich, N. Meidinger, V. Burwitz (MPE), E. Kuulkers, Andrea Santovincenzo (ESA), P. O'Brien (UoL) and B. Cordier (CEA) on behalf of the Einstein Probe team Following the detection of the fast X-ray transient EP240315a (Zhang et al., GCN 35931) and its optical counterpart (Srivastav et al., GCN 35932, 35933; Jiang et al., GCN 35935; Saccardi et al., GCN 35936; Rau GCN 35937; Chen GCN 35938; Lipunov, GCN 35940; Leonini et al., GCN 35944; Pankov et al., GCN 35945), we performed observations of EP240315a with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe mission. The observations began at 2024-03-17T14:10:00, about 42 hours after the EP-WXT detection. The exposure time is 3758 seconds. An X-ray afterglow was detected at R.A. = 141.6483 deg, DEC = -9.5335 deg, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), 3.2 arcsec away from the position of the optical counterpart AT2024eju (Srivastav et al., GCN 35932). The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 4.2E20 cm-2 and a photon index of 1.4(-0.5, +0.5). The derived flux in 0.3-10 keV is 1.8(-0.7, +1.4)E-13 erg/s/cm2 (90% C.L.). There is no cataloged X-ray source within the error circle, consistent with this source being the afterglow of EP240315a. Daily monitoring of the source with FXT is planned over the next few days. 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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES. View this GCN Circular online at https://gcn.nasa.gov/circulars/35951. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...