GCN Circular 36086

T. Y. Lian, X. Pan, Z. X. Ling, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, 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, 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), J. Yang, C. Y. Dai (NJU), Z. T. Fang (PMO), 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 EP240413a at 2024-04-13T14:39:37 (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. = 228.794 deg, DEC = -18.800 deg (J2000) with an uncertainty of 20 arcmin in radius (90% C.L. statistical and systematic).  The larger positioning uncertainty than the nominal value is due to the so far incomplete calibration of the WXT module which detected the source. The transient event lasts for ~200 seconds and has a peak flux of ~7 x 10^-9 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 1.6(-0.2/+0.1) (with the column density fixed at the Galactic value of 1.12 x 10^21 cm^-2), giving an unabsorbed flux of 1.1(-0.1/+0.1) x 10^-9 erg/s/cm2 in the 0.5-4 keV band. The 1-sigma uncertainties are given for the above parameters. We note that the derived source parameters may be subject to larger uncertainties than those quoted here since in-orbit calibration of the instrument is still in progress.
 
No previously known X-ray sources at a similar flux level are found within the 20 arcmin region around the source position.
 
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.