GCN Circular 36053

J. W. Hu, D. H. Zhao, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. 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, 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), 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 EP240408a at 2024-04-08T17:56:30 (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. = 158.840 deg, DEC = -35.749 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the source shows a duration of approximately 10 seconds. The peak flux reaches ~1.4 x 10^-8 erg/s/cm^2 in the 0.5-4.0 keV band. The averaged spectrum can be fitted by an absorbed power-law with NH fixed at the Galactic value of 6.23 x 10^20 cm^-2 and a photon index of 1.1(-0.7/+0.8). The derived average unabsorbed 0.5-4.0 keV flux is 4.0(-1.3/+1.3) x 10^-9 erg/s/cm^2. 

No previously known bright X-ray sources have been found within the 3 arcmin region around the source position. 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. 

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

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.