GCN Circular 36138

H. Q. Cheng, W. X. Wang, W. Yuan, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, 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 EP240416a at 2024-04-16T02:42:13 (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. = 203.150 deg, DEC = -13.612 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The flare lasted for at least 200 seconds, with the tail of the fading light curve undetected due to the interruption of the observation. The peak flux reached ~1.3 x 10^-9 erg/s/cm^2 in the 0.5-4.0 keV band. The averaged 0.5-4.0 keV spectrum can be fitted with an absorbed power law (with the column density fixed at the Galactic value of 4.0 x 10^20 cm^-2) with a photon index of 1.5(-0.6/+0.6). The derived average unabsorbed 0.5-4.0 keV flux is 5.0(-1.6/+2.6) x 10^-10 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
 
No previously known bright X-ray sources are found within the error circle around the source position. The derived source parameters may be subject to larger uncertainties than those quoted here since the in-orbit calibration of the instrument is still in progress.

Further follow-up observations are encouraged to identify the nature of this X-ray transient.

The above observation was made with EP-WXT during the commissioning phase. 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.