GCN Circular 37034

Y. Wang (PMO, CAS), H. Sun, Y. L. Wang, W. D. Zhang, Y. Liu, H. W. Pan, Z. X. Ling, C. C. Jin, 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, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, X. Pan, W. X. Wang, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, W. Yuan, M. 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 EP240804a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient started at 2024-08-04T21:36:53(UTC) and triggered the WXT on-board processing unit (trigger ID: 01709017305). The WXT position of EP240804a is R.A.= 337.644 deg, DEC = -39.121 deg (J2000) with an uncertainty of 4 arcmin (radius, 90% C.L. statistical and systematic). The lightcurve of the transient observed by the WXT lasts more than 100 seconds. The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 0.7(-0.4, +1.2) (with a freely fitted column density value of 1.1(-1.0, +37.0) x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 6.1(-1.6, +2.6) x 10^(-10) erg/s/cm^2. 

The EP-FXT automonous follow-up observation started at 2024-08-04T21:41:15 (UTC). The observation clearly shows an uncatalogued X-ray source located at R.A. = 337.6638 deg, DEC = -39.1243 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The FXT light curve shows significant variability. The average 0.5-10 keV spectrum can be fitted by an absorbed power law with a photon index of 1.6(+/-0.1) (with a free column density value of 2.8(+/-2.7) x 10^20 cm^-2), giving an average unabsorbed flux of 2.2(+/-0.1) x 10^(-11) erg/s/cm^2. The 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 X-ray follow-up capability (Yuan et al. 2022, Handbook of X-ray and Gamma-ray Astrophysics).