TITLE: GCN CIRCULAR NUMBER: 37188 SUBJECT: EP240816a: EP detection of a fast X-ray transient and follow-up observation DATE: 24/08/16 15:56:10 GMT FROM: EP Team at NAOC/CAS 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 EP240816a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient event started at 2024-08-16T03:28:42 (UTC) and triggered the WXT on-board processing unit (trigger ID: 01709034199). The light curve lasts for more than 200 seconds. The WXT position of the source is R.A. = 292.925 deg, DEC = -54.412 deg (J2000) with an uncertainty of 2.66 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 1.1(+0.6/-0.5) (with a frozen column density value of 4.2 x 10^20 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 3.0(+0.5/-0.5) x 10^-10 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. An automated observation by the Follow-up X-ray Telescope (FXT) on board EP was triggered at 2024-08-16T05:57:58 (UTC). Three uncatalogued X-ray sources, a relatively bright one and two faint sources, are detected within the error circle of the WXT source position. Their parameters derived from the data taken with the two identical FXT units (FXT-A and FXT-B) are summarized in the table. For the brightest source, EP J193152.4-542420, the average FXT spectrum in 0.5-10 keV band can be fitted by an absorbed power law with a photon index of 1.87(+0.40/-0.39) (with a frozen column density value of 4.2 x 10^20 cm^-2), giving an average unabsorbed flux in 0.5-10 keV of 1.2(+0.5/-0.3) x 10^-13 erg/s/cm^2. Given this source is significantly brighter than the other two, we tentatively suggest it to be likely associated with the transient EP240816a detected by WXT. However, it cannot be ruled out that the transient is associated with one of the other two faint FXT sources. Further follow-up observations are encouraged to identify the nature of this X-ray transient. ------------------------------------------------------------------------------------- Source | R.A. | Dec. | FXT-A Significance | FXT-B Significance | Estimated Flux | (J2000) | (J2000) | | | (erg/s/cm^2) ------------------------------------------------------------------------------------- EP J193152.4-542420 | 292.9686 | -54.4060 | 7.990 | 12.836 | 1.2 x 10^-13 EP J193128.6-542502 | 292.8693 | -54.4173 | 3.560 | - | 4.7 x 10^-14 EP J193132.7-542607 | 292.8862 | -54.4353 | - | 3.788 | 2.1 x 10^-14 ------------------------------------------------------------------------------------- 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).