TITLE: GCN CIRCULAR NUMBER: 36818 SUBJECT: EP240703c: EP-WXT detection of an X-ray transient DATE: 24/07/04 07:01:58 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn
Y. J. Zhang (THU), M. J. Liu, S. Q. Jiang, H. W. Pan, C. C. Jin, Z. X. Ling, W. Yuan, Y. Liu, C. Zhang, W. Chen, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, D. Y. Li, H. Y. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, X. Pan, H. Sun, W. X. Wang, Y. L. Wang, S. X. Wen, Q. Y. Wu, X. P. Xu, Y. F. Xu, H. N. Yang, 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 an X-ray transient EP240703c at 2024-07-03T18:15:00 (UTC) by the Wide-field X-ray Telescope (WXT) on board the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 289.264 deg, DEC = -30.325 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The source lasted for over 1 ks with multipeak structure in its lightcurve. The average 0.5-4 keV spectrum can be fitted with an absorbed power law (with the column density fixed at the Galactic value of 8 x 10^20 cm^-2) with a photon index of 1.3(-0.5/+0.5). The unabsorbed flux is 2.5(-0.8/+0.8) x 10^-10 erg/s/cm^2 in the 0.5-4 keV band. The 90 per cent uncertainties are given for the above parameters.
Within the error circle of the WXT position, there is a previous X-ray observation of a high proper motion star LP 924-17 with a flux of 4.6 x 10^-13 erg/cm2/s in the 0.3-10 keV range. Based on the light curve and spectrum of the X-ray transient, we tend to consider that the source is not a stellar flare, although the possibility cannot be ruled out.
We have proposed a Swift target of opportunity observation. Further follow-up observations are encouraged to identify the nature of this X-ray transient.
The above observation was made with the WXT instrument 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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