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GCN Circular 36129

EP240414a: EP-FXT follow-up observation
2024-04-16T04:05:44Z (3 months ago)
EP Team at NAOC/CAS <>
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J. Guan, C. K. Li, Y. Chen, S.M. Jia, W. W. Cui, H. Feng, 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 EP240414a (Lian et al., GCN 36091) 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 2024-04-14T11:50:01, about 2 hours after the WXT detection, with a net exposure time of 7.2 ks. 

An X-ray source was clearly detected at R.A. = 191.509 deg, Dec. = -9.718 deg, which is R.A. = 12:46:02.16, Dec. = -09:43:04.8, with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). We note that the source is ~8 arcsec away from the optical transient (OT) which is decaying slowly (Aryan et al., GCN 36094; Xu et al., GCN 36105; Jonker et al.,  GCN 36110). There is no cataloged X-ray source within the error circle. Considering this FXT detected source being within the error circle of the WXT source (1.5 arcmin away) and being likely associated with the OT, we tend to suggest it to be the late-time X-ray emission from EP240414a. The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 3.35e20 cm-2 and a photon index of 1.7(+/-0.3) (90% C.L.). The derived unabsorbed flux in 0.5-10 keV is 3.5(+/-0.8)e-13 erg/s/cm2 (90% C.L.). Assuming this source is indeed associated with EP240414a, its X-ray flux has faded by about 4 orders of magnitude in X-ray flux in about 2 hours since the WXT detection. 
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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