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GCN Circular 36514

Subject
EP240518a: possibly identified with a flare star by EP-FXT
Date
2024-05-19T16:22:25Z (25 days ago)
From
EP Team at NAOC/CAS <ep_ta@bao.ac.cn>
Via
Web form
Q.Y. Wu (NAOC, CAS), A. Li (BNU), Y. Liu, W. Yuan, Z. X. Ling, C. Zhang, C. C. Jin, H. Q. Cheng, W. Chen, C. Z. Cui, D. W. Fan, H. B. Hu, J. W. Hu, M. H. Huang, H. Y. Liu, M. J. Liu, Z. Z. Lv, T. Y. Lian, X. Mao, H. W. Pan, H. Sun, W. X. Wang, Y. L. Wang, X. P. Xu, Y. F. Xu, H. N. Yang, M. Zhang, W. D. Zhang, W. J. Zhang, Z. Zhang (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

Following the detection of the X-ray transient EP240518a (Wu et al., GCN 36512) 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 2024-05-18T15:46:11, about 3 hours after the WXT detection, with a net exposure time of 6.0 ks.

Within the error circle of EP240518a, an X-ray source was clearly detected at R.A. = 216.954 deg, DEC = -49.584 deg, with an uncertainty of 30 arcsec (radius, 90% C.L. statistical and systematic). It is 7.9 arcsec away from a known X-ray source 1eRASS J142749.6-493505 with an archived flux of 6.0 x 10^-14 erg/s/cm^2 in 0.2-2.3 keV. The absorbed flux of the FXT source is 1.1(+0.3/-0.2) x 10^-13 erg/s/cm^2 (90% C.L.) in 0.5-10 keV, which is generally consistent with the eRASS flux but is about 3 orders of magnitude lower than the peak flux of EP240518a (~8 x 10^-11 erg/s/cm^2). This suggests that the time scale of EP240518a, if associated with the FXT source, is shorter than several hours. The FXT spectrum can be well fitted with the tbabs*apec model with the plasma temperature kT = 0.21(+0.10/-0.06) keV (90% C.L.). A possible M-type dwarf star named Gaia DR3 5899529595616014848, with a distance of about 35 pc, is located around 7 arcsec from the FXT position. If the FXT source is related to the eRASS source and the dwarf, then EP240518a may be a stellar flare with a luminosity of 1.3 x 10^31 erg/s in 0.5-4.0 keV.

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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