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GCN Circular 36161

EP240417a: EP-WXT detection of a fast X-ray transient
2024-04-18T16:56:42Z (a month ago)
EP Team at NAOC/CAS <>
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J. W. Hu, W. Chen, S. Q. Jiang, C. C. Jin, Y. Liu, Z. X. Ling, C. Zhang, H. Q. Cheng, C. Z. Cui, D. W. Fan, H. B. Hu, M. H. Huang, D. Y. Li, T. Y. Lian, H. Y. Liu, M. J. Liu, Z. Z. Lv, X. Mao, H. W. Pan, X. Pan, H. Sun, 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), 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 EP240417a at 2024-04-17T15:12:33 (UTC) by the Wide-field X-ray Telescope (WXT) onboard the Einstein Probe (EP) mission during a calibration observation. The position of the source is R.A. = 177.442 deg, DEC = -15.438 deg (J2000) with an uncertainty of 3 arcmin (radius, 90% C.L. statistical and systematic). The flare lasted for at least 1500 seconds, with the tail of the fading light curve undetected due to the interruption of the observation. The peak flux reached ~3 x 10^-10 erg/s/cm^2 in the 0.5-4.0 keV band. The averaged 0.5-4.0 keV spectrum can be well fitted with a single power law of photon index 1.15(-0.26/+0.27), and the best-fit absorption column density is consistent with 0. The derived average 0.5-4.0 keV flux is 9.8(-0.8/+0.8) x 10^-11 erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters.
No previously known bright X-ray sources are found within the error circle around the source position. The derived source parameters may be subject to larger uncertainties than those quoted here since the in-orbit calibration of the instrument is still in progress.

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

The above observation was made with EP-WXT during the commissioning phase. 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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