TITLE: GCN CIRCULAR NUMBER: 36330 SUBJECT: EP240426b: EP-WXT detection of a new fast X-ray transient DATE: 24/04/27 05:01:10 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> X. Pan, D. H. Zhao (NAOC, CAS), J. Q. Peng (IHEP, CAS), C. C. Jin, Z. X. Ling, Y. Liu, Z. X. Ling, C. Zhang, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, 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 (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 EP240426b at 2024-04-26T14:19:06 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 173.787 deg, DEC = -40.741 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for ~300 seconds and has a peak flux of ~9.5 x 10^-10 erg/s/cm^2 in the 0.5-4 keV band. The averaged 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.8(-0.3/+0.3) (with the column density fixed at the Galactic value of 8.08 x 10^20 cm^-2), giving an unabsorbed flux of 3.6(-0.6/+0.6) x 10^-10 erg/s/cm2 in the 0.5-4 keV band. The 1-sigma uncertainties are given for the above parameters. Note: EP240426b is about 40 degrees away from the 90% credible region of the recent BH-NS gravitational-wave event S240422ed (GCN 36236), therefore it is an independent source unrelated to this event. No previously known X-ray sources at a similar flux level are found within the 3 arcmin region around the source position. Further multi-wavelength follow-up observations are encouraged to identify the nature of this X-ray flare. 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. View this GCN Circular online at https://gcn.nasa.gov/circulars/36330. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...