TITLE:   GCN CIRCULAR
NUMBER:  37492
SUBJECT: EP240913a: EP-WXT detection of a fast X-ray transient
DATE:    24/09/14 01:40:12 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

D. Y. Li, X. P. Xu (NAOC, CAS), B. T. Wang (YNAO, CAS), Y. J. Zhang (THU), D. F. Hu (PMO, CAS), C. C. Jin, Z. X. Ling, W. D. Zhang, 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, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, Q. Y. Wu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), Y. Chen, S. M. Jia, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, X. F. Zhao, J. Zhang, 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 EP240913a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at T0=2024-09-13T11:39:33(UTC). The WXT position of EP240913a is R.A.= 16.681 deg, DEC = 16.750 deg (J2000) with an uncertainty of 2.5 arcmin (radius, 90% C.L. statistical and systematic). At ~T0+180s, the light curve shows a fast pulse lasting around 50s, followed by a week emission up to 1100s. It has a peak flux of ~2 x 10^-9 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 0.5(-0.6, +0.6) (with the absorption fixed at the Galactic value of 5.1 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.1(-0.4, +0.5) 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. Follow-up observations by EP-FXT and Swift will be requested, and further observations are encouraged to identify the nature of this X-ray transient. 

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).