GCN Circular 36838

W. Chen, Y. L. Wang, J. W. Hu, C. C. Jin, Z. X. Ling, Y. Liu, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, T. Y. Lian, Z.Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. 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), 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 EP240708a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-07-08T23:28:23 (UTC) and triggered the WXT on-board processing unit. The position of the source is R.A. = 345.963 deg, DEC = -22.840 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 1300 seconds and has a peak flux of ~1.1 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.57(+0.63/-0.54) with the column density fixed at the Galactic Absorption of 2.0 x 10^20 cm^-2. The derived average unabsorbed 0.5-4 keV flux is 7.7(+3.4/-2.6) x 10^-11 erg/s/cm^2. The uncertainties are given at the 90% confidence level 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.

Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP and proposed a Swift target of opportunity observation. Further follow-up observations are 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.