GCN Circular 37019

Y. L. Wang (NAOC, CAS), A. Li (BNU), W. X. Wang, H. W. Pan, Z. X. Ling, C. C. Jin, Y. Liu, C. Zhang, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, 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, 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 EP240802a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. A follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was triggered about 14 hours after the WXT detection, and an uncatalogued X-ray source is detected at R.A. = 287.8070 deg, DEC = -2.3125 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). 

The transient event was observed to start at 2024-08-02T10:32:52 (UTC). The WXT position of the source is R.A. = 287.802 deg, DEC = -2.354 deg (J2000) with an uncertainty of 1.9 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for more than 500 seconds. The lightcurve of the transient observed by the WXT exhibits three sequentially weakened peaks. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 0.94(+0.59/-0.56) (with a freely fitted column density value of 2.5(+2.0/-1.8) x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.7(+0.4/-0.3) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
 
The average FXT spectrum in 0.5-10 keV band can be fitted by an absorbed power law with a photon index of 1.89(+0.33/-0.31) (with a freely fitted column density value of 4.8(+1.4/-1.2) x 10^21 cm^-2), giving an average unabsorbed flux of 1.3(+0.2/-0.1) x 10^-12 erg/s/cm^2.
 
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 X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).