TITLE: GCN CIRCULAR NUMBER: 36319 SUBJECT: LIGO/Virgo/KAGRA S240422ed: EP-FXT Detection of the Potential Opitcal Electromagnetic Counterpart AT 2024hfq Reported by DECam DATE: 24/04/26 16:02:40 GMT FROM: EP Team at NAOC/CAS ep_ta@bao.ac.cn

C.K. Li (IHEP, CAS), D. Y. Li, H. Q. Cheng, J. W. Hu, H. Sun, C. C. Jin (NAOC, CAS), J. Guan, H. S. Zhao, Y. Chen, S.M. Jia, W. W. Cui, J. Zhang, H. Feng, W. Li, C. Z. Liu, F. J. Lu, L. M. Song, J. Wang, J. J. Xu, D. W. Han, S. N. Zhang, 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), B. Zhang (UNLV), W. D. Zhang, Z. X. Ling, Y. Liu, H. Y. Liu, Y. F. Xu, W. Yuan, M. Zhang, C. Zhang (NAOC, CAS) on behalf of the Einstein Probe team

We report the results from a follow-up observation with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP) mission, on one of the potential optical counterparts, namely AT 2024hfq, as reported by DECam (Cabrera et al. GCN #36298).

Our EP-FXT observation started at 2024-04-25 11:57:54 (UTC) with an exposure time of 1189 seconds. An X-ray source was signifcantly detected at the position of R.A. = 126.2886, DEC. = -24.7273 (R.A. = 08:25:09.26, DEC. = -24:43:38.3), with a position uncertainty of 10" (90% C.L. statistical and systematic). This position is 9.4 arcsec away from AT 2024hfq, and 9.7 arcsec away from the nearby galaxy WISEA J082508.82-244345.9. The absorption-corrected flux in 0.5-10 keV is about 2.7 x 10 ^-13 erg/s/cm2 assuming a power-law spectrum with a photon index of 2.0 with Galactic absorption. No previously known X-ray sources are found within the error circle around this position. A flux upper limit set by the eRASS1 survey is 2.4 x 10^-13 erg/s/cm2 (99.9% C.L.) in 0.2-5.0 keV .

Please note that EP-FXT is currently undergoing in-flight calibration, and the derived source parameters may be subject to larger uncertainties than quoted here; 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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