GCN Circular 37214

Y. F. Liang (PMO, CAS), H. Y. Liu, X. Mao, C. C. Jin, 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), 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 EP240820a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-08-20T00:54:47(UTC). The WXT position of EP240820a is R.A.= 16.221 deg, DEC = -34.698 deg (J2000) with an uncertainty of 2.1 arcmin (radius, 90% C.L. statistical and systematic). The light curve of the transient observed by the WXT lasts around 250 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.2(-0.7, +0.8) (with a column density fixed at the Galactic one of 1.62 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.2(-0.5, +0.7) x 10^(-10) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP. The observation began at 2024-08-20T09:48:47 (UTC), and the exposure time is about 1.8 ks. FXT-B detected an uncataloged weak souce at R.A. = 16.2459 , DEC = -34.6941 (J2000) with an uncertainty of about 10 arcsec (radius, 90% C.L. statistical and systematic). Given the limited counts, the FXT spectrum is fitted using an absorbed power law and the parameters are fixed to those obtained by the EP-WXT spectral fitting. The derived flux is about 2.0 x 10^(-13) erg/s/cm^2. 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).