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GCN Circular 36203

EP240420a: detection of fading X-ray emission with refined position by EP-FXT
2024-04-21T05:26:43Z (3 months ago)
EP Team at NAOC/CAS <>
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D. W. Han (IHEP, CAS), W. J. Zhang (NAOC, CAS), C. K. Li, Y. Chen, S. M. Jia, W. W. Cui, J. Guan, H. Feng, 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), P. Friedrich, V. Burwitz, N. Meidinger, K. Nandra, A. Rau (MPE), B. Cordier (CEA), S. X. Wen, W. X. Wang, 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 EP240420a (Zhang et al., GCN 36194) by the Wide-field X-ray Telescope (WXT) on board Einstein Probe (EP), we performed an observation with EP's Follow-up X-ray Telescope (FXT). The observation started at 2024-04-20T14:05:01, about 2 hours after the WXT detection of the X-ray flare, with a net exposure time of 7.1ks.

An X-ray source was clearly detected at R.A. = 228.7291 deg, Dec. =14.8024 deg (R.A. = 15:14:54.98, Dec = 14:48:08.6), with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic), which is 1 arcmin away from the reported WXT position of EP240420a. There is no cataloged X-ray source within the error circle. The FXT count rate of the source has dropped by a factor of 50% over the observation duration of 14 ks. Considering this source being within the error circle of the WXT transient and being fading away, we suggest it being the late-time X-ray emission from EP240420a.The spectrum can be fitted with an absorbed power-law model with NH fixed at the Galactic value of 2.5e20 cm-2 and a photon index of 1.9(+/-0.1). The derived unabsorbed flux in 0.5-10 keV is 1.2(+/-0.1)e-12 erg/s/cm2. Assuming this source is indeed associated with EP240420a, its X-ray flux has faded by about more than 3 orders of magnitude in X-ray flux since the WXT detection. The quoted errors above are at the 90% confidence level. 

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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