TITLE: GCN CIRCULAR NUMBER: 42818 SUBJECT: LIGO/Virgo/KAGRA S251105aj: EP-FXT follow-up observations and preliminary results on SVOM J2320.0-2901 DATE: 25/11/24 15:05:57 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> S. Guillot, H. Yang, O. Godet, J.-L. Atteia (IRAP), M. Pillas (IAP), L. P. Xin, Y. N. Ma, H. W. Pan, T. Zhao (NAOC), Y. J. Yi, A. Li (BNU), R. X. Hu (WHU), D. Z. Du (ZJU), C. Y. Wang (THU) on behalf of the Einstein Probe (EP) team: EP-FXT performed a Target of Opportunity observation of the high-energy transient SVOM J2320.0-2901 detected by SVOM/ECLAIRs (Yang et al., GCN 42797), in a search for possible counterparts of the compact binary merger candidate S251105aj (The LIGO-Virgo-Kagra Collaboration, GCN 42587). The observation was conducted at 2025-11-15T21:39:22 UTC with an exposure of 5 ks, about 10.36 days after the GW trigger and the detection time of SVOM J2320.0-2901. EP-FXT covered the full area of the 90% C.L. ECLAIRs error region and detected six sources above a SNR of 4.5 within 15 arcminutes from SVOM J2320.0-2901. Their information is summarized below (RA/Dec in degrees with 10" error radius (90% C.L.); fluxes are the observed ones in 0.5–10 keV): # | Name | RA | Dec | Flux (erg/s/cm^2) | SNR —---------------------------------------------------------------------------------------------------- 1 | EPF_J231927.4-290337 | 349.8643 | -29.0602 | 1.4e-13 | 4.7 2 | EPF_J232034.0-290053 | 350.1419 | -29.0148 | 2.2e-13 | 10.0 3 | EPF_J231953.7-285108 | 349.9737 | -28.8522 | 1.7e-13 | 5.7 4 | EPF_J231906.2-290806 | 349.7760 | -29.1351 | 3.4e-13 | 5.8 5 | EPF_J232051.4-285356 | 350.2140 | -28.8990 | 2.4e-13 | 8.4 6 | EPF_J232108.1-290032 | 350.2839 | -29.0088 | 2.6e-13 | 7.9 We cross-matched all six FXT detections against the SIMBAD, NED, and Vizier catalogues, using a 20″ matching radius (for a galaxy at 2000 Mpc, this corresponds to an offset of ~200 kpc). Five FXT detections (#2,3,4,5,6) are matched to five optical quasars correspondingly, with their redshifts in a range of 0.721 to 1.641, higher than z=0.374 estimated for the GW event. The remaining source (#1) is matched to a source from the KiDSDR4 QSOs photometric redshifts catalog (Nakoneczny et al., 2021), which is classified as a quasar with 68% probability and a photometric redshift of 0.5 +/- 1.2 based on machine learning estimates. We conducted SVOM/VT on this field with the results reported in GCN 42819. Further FXT observations of these candidates will be conducted. 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). View this GCN Circular online at https://gcn.nasa.gov/circulars/42818. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...