GCN Circular 36997

H. Zhou (PMO, CAS), W. X. Wang, J. W. Hu, 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, 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, W. J. 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 

We report on the detection of a fast X-ray transient (trigger ID: 01709015181), designated EP240801a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2024-08-01T09:06:03 (UTC). The position of the source is R.A. = 345.140 deg, DEC = 32.610 deg (J2000) with an uncertainty of 2.4 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for more than 80 seconds. The lightcurve of the transient observed by the WXT exhibits a rapid brightening profile. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.9(+/-0.9) (with a free column density value of 3.1(+/-3.1) x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 4.8(+/-3.1) x 10^-10 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

The autonomous EP-FXT follow-up observation started about 180s after the WXT trigger, and the observation clearly shows an uncatalogued X-ray source located at R.A. = 345.1630 deg, DEC = 32.5927 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The 0.5-10 keV light curve of the transient decays smoothly and lasts about 200s since the start time of the autonomous follow-up observation. The average 0.5-10 keV spectrum can be fitted by an absorbed power law with a photon index of 3.2(+/-0.4) (with a free column density value of 1.5(+/-0.7) x 10^21 cm^-2), giving an average unabsorbed flux of 1.5(+/-0.3) x 10^-11 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).