TITLE:   GCN CIRCULAR
NUMBER:  37561
SUBJECT: EP240919a: EP-WXT detection of a fast X-ray transient
DATE:    24/09/20 07:39:06 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

Y. F. Liang (PMO, CAS), H. Q. Cheng, M. J. Liu, X. Pan, X. P. Xu, H. N. Yang(NAOC, CAS), Z. J. Zhang (HKU), C. Y. Dai (NJU), C. X. Zhang (HUST), C. C. Jin, H. W. Pan, Z. X. Ling, W. M. Yuan, Y. Liu, C. Zhang, W. Chen, , C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. Sun, W. X. Wang, Y. L. Wang, Y. F. Xu, 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), 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 EP240919a, which was detected by EP-WXT at 2024-09-19T14:47:40 (UTC), and triggered the on-board processing unit at 2024-09-19T14:53:43 (trigger ID: 01709054141). An autonomous observation was performed by the Follow-up X-ray Telescope (FXT) about five minutes later. Afterwards, the FXT on-board processing unit was also triggered, showing that an uncatalogued source was detected at R.A. = 334.2797, DEC = -9.7362 (J2000) with an uncertainty of about 20 arcsec (radius, 90% C.L. statistical and systematic). The light curve of the transient observed by the WXT lasts more than 400 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed power-law with a photon index of 1.3(+0.3/-0.3) (with the column density fixed at the Galactic value of 5.4x10^20 cm^-2). The absorbed average flux in 0.5-4 keV is 7.0(+1.8/-1.5) x 10^(-10) erg/s/cm^2. The quoted 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. 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 onboard X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).