TITLE: GCN CIRCULAR NUMBER: 36690 SUBJECT: EP240618a: EP-WXT detection of a fast X-ray transient and EP-FXT quick follow-up DATE: 24/06/18 12:34:18 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

H. Sun, W. Chen (NAOC, CAS), H. Zhou (PMO, CAS), W. D. Zhang, J. W. Hu, D. Y. Li, Y. Liu, Z. X. Ling, C. Zhang, C. C. Jin, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, 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. D. Zhang, W. J. Zhang, Z. Zhang, D. H. Zhao (NAOC, CAS), J. Yang, C. Y. Dai (NJU) Y. F. Liang (PMO), 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, designated EP240618a, by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission. The transient triggered the WXT on-board processing unit at 2024-06-18T05:43:43 (UTC). The position of the source is R.A. = 281.627 deg, DEC = 23.820 deg (J2000) with an uncertainty of 3 arcmin in radius (90% C.L. statistical and systematic). The transient event lasts for approximately 100 seconds and has a peak unabsorbed flux of ~1.5 x 10^-8 erg/s/cm^2 in the 0.5-4 keV band. The average 0.5-4 keV spectrum can be fitted by an absorbed power law with a photon index of 1.2(+/-0.4) (with the column density fixed at the Galactic value of 1.98 x 10^21 cm^-2). The derived average unabsorbed 0.5-4 keV flux is 2.9(+0.7/-0.6) x 10^-9 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. No previously known X-ray sources at a similar flux level are found within the 3 arcmin region around the source position.

Following the WXT detection, we performed a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the EP, which started at 2024-06-18T06:55:00 (UTC), about 1 hour after the WXT detection. An X-ray source was clearly detected at R.A. = 281.648 deg, DEC = 23.833 deg (R.A. = 18:46:35.4, Dec = 23:49:57.0; J2000), with an uncertainty of 30 arcsec (radius, 90% C.L. statistical and systematic), which is 1.4 arcmin away from the WXT position of EP240618a. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 1.98 x 10^21 cm^-2 and a photon index of 2.0(+/-0.1). The derived unabsorbed flux in 0.5-10 keV is 3.0(+/-0.3) x 10^-12 erg/s/cm2. During the observation, the flux of the source was decaying with time. Considering this source being within the error circle of the WXT transient, we suggest it being the late-time X-ray emission from EP240618a.

Further follow-up observations are encouraged to identify the nature of this X-ray transient.

The above observations were made with the WXT and FXT instruments during the commissioning phase of EP. 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). EP is a mission of the Chinese Academy of Sciences in collaboration with ESA, MPE and CNES.

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