GCN Circular 36405

D.Y. Li (NAOC, CAS), J. Yang (NJU), A. Li (BNU), W. Yuan, Z. X. Ling, Y. Liu, C. Zhang, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, 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, 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, designated EP240506a, at 2024-05-06T05:01:39 (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. = 213.978 deg, DEC = -16.715 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for about 50 seconds and has a peak flux of 1 x 10^-8 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 1.0(+/- 0.5) (with the column density fixed at the Galactic value of 9.0 x 10^20 cm^-2), giving an unabsorbed flux of 1.1(+0.3, -0.2) x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The 1-sigma uncertainties are given for the above parameters.
 
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.