TITLE: GCN CIRCULAR NUMBER: 38345 SUBJECT: LIGO/Virgo/KAGRA S241125n: EP-FXT follow-up observations DATE: 24/11/27 14:24:16 GMT FROM: EP Team at NAOC/CAS <ep_ta@bao.ac.cn> Y. L. Wang, S. X. Wen, W. X. Wang (NAO, CAS), B.-T. Wang (YNAO, CAS), T. C. Zheng (PMO, CAS), W. Yuan, D. Y. Li, Y. Liu (NAO, CAS), B. Zhang (UNLV) report on behalf of the Einstein Probe team: Following on the trigger of the detection of the gravitational-wave (GW) event S241125n (LIGO/Virgo/KAGRA collaborations, GCN 38305) and the Swift/BAT-GUANO detection of a candidate counterpart (DeLaunay et al., GCN 38308), we performed a target-of-opportunity observation of the BAT position (RA, Dec = 58.079, +69.689 deg) with the Follow-up X-ray Telescope (FXT) on board the Einstein Probe (EP). The observation started at 2024-11-26 03:08:45 (UTC), about 26 hours after the GW event, with an exposure of ~ 11 ks. Within the 5-arcmin error circle of Swift/BAT (50% containment; DeLaunay et al., GCN 38308), one X-ray source is detected by both modules of the EP-FXT, at an averaged position at R.A. = 58.1097 deg, DEC = 69.6392 deg (J2000), with an uncertainty of 10 arcsec (radius, 90% C.L. statistical and systematic). The average 0.5-10 keV spectrum can be fitted with an absorbed power law with a photon index of 0.43 (+0.76/-0.74) (with a column density fixed at the Galactic value of 3.4 x 10^21 cm^-2). The derived average unabsorbed 0.5-10 keV flux is 1.17 (+1.18/-0.63) x 10^(-13) erg/s/cm^2. The uncertainties are at the 90% confidence level for the above parameters. The location of this source is consistent with S241125n_X3, an uncatalogued X-ray source detected by Swift/XRT (Page et al., GCN 38324). Outside the 5-arcmin error circle of the Swift/BAT source position, EP-FXT detected several other X-ray sources, some of which are consistent with the sources reported by Swift/XRT (Page et al., GCN 38324). We list the sources within the 10-arcmin radius in the followed two tables for FXT-A and FXT-B, respectively, with the seperations from the BAT source given. The observed 0.5-10 keV flux given in the tables is a rough estimate assuming the same spectral paramters as quoted before, and is not corrected for absorption. | FXT-A source | R.A. (deg, J2000) | DEC (deg, J2000) | Sep. (arcmin) | Flux (erg/s/cm^2) | | EPF_J035226.4+693824*| 58.1096 | 69.6383 | 3.11 | 1.05 x 10^(-13) | | EPF_J035109.4+693830*| 57.7917 | 69.6411 | 6.64 | 1.52 x 10^(-13) | | EPF_J035330.7+693812 | 58.3826 | 69.6391 | 7.00 | 1.64 x 10^(-13) | | EPF_J035104.0+693411 | 57.7667 | 69.5698 | 9.68 | 2.34 x 10^(-13) | | EPF_J035318.9+693257*| 58.3287 | 69.5492 | 9.88 | 1.28 x 10^(-13) | | EPF_J035113.2+694926*| 57.8023 | 69.8230 | 9.88 | 5.84 x 10^(-13) | | FXT-A source | R.A. (deg, J2000) | DEC (deg, J2000) | Sep. (arcmin) | Flux (erg/s/cm^2) | | EPF_J035226.4+693824*| 58.1098 | 69.6401 | 3.00 | 1.87 x 10^(-13) | | EPF_J035109.4+693830*| 57.7891 | 69.6418 | 6.68 | 2.45 x 10^(-13) | | EPF_J035330.7+693812 | 58.3779 | 69.6368 | 6.98 | 1.52 x 10^(-13) | | EPF_J035044.0+694519 | 57.6832 | 69.7554 | 9.14 | 2.45 x 10^(-13) | | EPF_J035113.2+694926*| 57.8051 | 69.8238 | 9.89 | 7.71 x 10^(-13) | | EPF_J035028.2+694407 | 57.6174 | 69.7353 | 10.00 | 1.17 x 10^(-13) | * sources also detected by Swift/XRT (Page et al., GCN 38324) Launched on January 9, 2024, EP is a space X-ray observatory to monitor the soft X-ray sky with onboard 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/38345. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...