TITLE:   GCN CIRCULAR
NUMBER:  36810
SUBJECT: EP240703b: EP-WXT detection of a fast X-ray transient
DATE:    24/07/03 15:09:49 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

J. Q. Peng, Q. C. Shui (IHEP, CAS), C. Y. Dai (NJU), Y. L. Wang, C. C. Jin, Z. X. Ling, W. Yuan, Y. Liu, C. Zhang, H. Q. Cheng, 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, T. Y. Lian, X. Mao, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, S. X. Wen, 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), 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 EP240703b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-07-03T05:24:26 (UTC). The position of the source is R.A. = 279.539 deg, DEC = -57.401 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The corresponding Galactic coordinates are l = 338.077, b = -20.878.

The transient lasted for approximately 600 seconds and had a peak absorbed flux of ~ 3 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 column density of 1.4(+1.3/-1.2) x 10^21 cm^-2 and a photon index of 1.5(+0.6/-0.5). The derived average unabsorbed 0.5-4 keV flux is 7.5(+1.3/-1.8) x 10^-10 erg/s/cm^2. If the column density is fixed at the Galactic value of 6.8 x 10^20 cm^-2, the derived photon index is 1.2(+/-0.2) and the average unabsorbed 0.5-4 keV flux is 7.1(+1.1/-1.0) x 10^-10 erg/s/cm^2. The uncertainties are 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. 

We have 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.