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GCN Circular 36092

EP240413a: possible detection of the X-ray emission with EP-FXT after 14 hours
2024-04-14T16:17:35Z (a month ago)
EP Team at NAOC/CAS <>
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C. K. Li, S.M. Jia, Y. Chen, W. W. Cui, H. Feng, J. Guan, D. W. Han, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, J. Zhang, S. N. Zhang, H. S. Zhao, X. F. Zhao (IHEP, CAS), E. Kuulkers, A. Santovincenzo (ESA), P. O'Brien (Univ. of Leicester), K. Nandra, A. Rau (MPE), B. Cordier (CEA), T. Y. Lian, X. Pan, Z. X. Ling, Y. Liu, H. Sun, D. Y. Li, H. Y. Liu, W. Yuan, C. Zhang (NAOC, CAS) on behalf of the Einstein Probe team

Following the detection of the fast X-ray transient EP240413a (Lian et al., GCN 36086) by the Wide-field X-ray Telescope (WXT) of Einstein Probe (EP), we performed an observation with the Follow-up X-ray Telescope (FXT) on board EP. The observation started at 2024-04-14T04:40:26, about 14 hours after the WXT detection, with a net exposure time of 9.3 ks. 

An X-ray source was clearly detected at R.A. = 228.815 deg, DEC = -18.503 deg, with an uncertainty of 30 arcsec (radius, 90% C.L. statistical and systematic). There is no cataloged X-ray source within the error circle. Considering this source being within the error circle of the WXT source (17.9 arcmin away), we tend to suggest it to be possibly associated with EP240413a. If this is true, EP240413a has faded by about 3 orders of magnitude in X-ray flux in about 14 hours. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 1.1e21 cm-2 and a photon index of 2.2(+/-0.3) (90% C.L.). The derived unabsorbed flux in 0.5-10 keV is 5.3(+/-0.5)e-13 erg/s/cm2 (90% C.L.). Further monitoring of EP240413a with FXT is planned for the next few days. 
Please note that EP-FXT is currently undergoing in-flight calibration. The derived source parameters may be subject to larger uncertainties, so please use them with caution. 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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