TITLE:   GCN CIRCULAR
NUMBER:  37648
SUBJECT: EP240930a: EP-WXT detection of a fast X-ray transient
DATE:    24/10/01 04:53:36 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

X. Tian (GXU), H. L. Peng (NNU), X. Mao, C. C. Jin, H. Y. Liu, Y. Liu, Z. X. Ling, C. Zhang, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W., Hu, M. H. Huang, D. Y. Li, M. J. Liu, Z. Z. Lv, T. Y. Lian, H. W. Pan, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, 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, W. W. Cui, D. W. Han, C. K. Li, L. M. Song, X. F. Zhao, J. Zhang, S. N. Zhang (IHEP, CAS), Y F. Liang (PMO, 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 EP240930a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-09-30T17:17:50(UTC) and lasted for around 200 seconds. The WXT position of EP240930a is R.A.= 319.899 deg, DEC = 41.303 deg (J2000) with an uncertainty of 2.1 arcmin (radius, 90% C.L. statistical and systematic). The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 0.79(-0.25, +0.24) (with a column density fixed at the Galactic one of 3.29 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 3.8(-0.5, +0.5) x 10^(-9) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. 

EP240930a may be associated with Fermi GRB 240930B (Fermi GBM team, GCN 37640) considering the similar trigger time and localization. No previously known bright X-ray sources are found within the error circle around the source position. A follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP is planned. Further follow-up 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).