TITLE:   GCN CIRCULAR
NUMBER:  37185
SUBJECT: EP240816b: EP-WXT detection and EP-FXT follow-up observation of a fast X-ray transient
DATE:    24/08/16 11:43:36 GMT
FROM:    EP Team at NAOC/CAS <ep_ta@bao.ac.cn>

H. N. Yang, W. J. Zhang, W. X. Wang, W. Yuan, Z. X. Ling, C. C. Jin, Y. Liu, 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, H. W. Pan, X. Pan, H. Sun, Y. L. Wang, Q. Y. Wu, X. P. Xu, Y. F. Xu, M. Zhang, W. D. 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 EP240816b, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2024-08-16T01:44:27 (UTC) and triggered the WXT on-board processing unit (trigger ID: 01709034033).
The light curve lasts for more than 50 seconds. The WXT position of the source is R.A. = 16.013 deg, DEC = 15.398 deg (J2000) with an uncertainty of 3.33 arcmin in radius (90% C.L. statistical and systematic). The average 0.5-4 keV spectrum can be fitted with an absorbed power law with a photon index of 1.6(+0.7/-0.7) (with a frozen column density value of 5.2 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 1.3(+0.7/-0.5) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.

The follow-up observation by the Follow-up X-ray Telescope (FXT) on board EP was triggered automatically at 2024-08-16T01:47:34 (UTC). An uncatalogued X-ray source is detected at R.A. = 16.0161 deg, DEC = 15.4151 deg (J2000) with an uncertainty of 10 arcsec in radius (90% C.L. statistical and systematic). The average FXT spectrum in 0.5-10 keV band can be fitted by an absorbed power law with a photon index of 1.69(+0.23/-0.22) (with a freely fitted column density value of 9.5(+6.9/-6.7) x 10^20 cm^-2), giving an average unabsorbed flux in 0.5-10 keV of 4.3(+0.6/-0.5) 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).