TITLE:   GCN CIRCULAR
NUMBER:  36194
SUBJECT: EP240420a: EP-WXT detection of a fast X-ray transient
DATE:    24/04/20 14:22:52 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

W. J. Zhang (NAOC, CAS), Y. J. Zhang (THU), S. X. Wen, W. X. Wang, Z. X. Ling, Y. Liu, C. Zhang, C. C. Jin, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. Zhang, W. D. 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
 
Subjects: X-ray, Transient
 
We report on the detection of a fast X-ray transient, designated EP240420a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient triggered the WXT on-board processing unit at 2024-04-20T12:04:28 (UTC). The position of the source is R.A. = 228.713 deg, DEC = 14.796 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The flare lasted for more than 100 seconds and reached a peak flux of ~1 x 10^-8 erg/s/cm^2 in the 0.5-4 keV band. Since only the basic data of the alert was transmitted to the ground, the information on the source property is limited. Further information may be provided once the full scientific telemetry data are received at the EP science center.
 
No previously known X-ray sources at a similar flux level are found within 3 arcmin around the source position.  
 
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.