TITLE: GCN CIRCULAR NUMBER: 44468 SUBJECT: GRB 260504B: SVOM/MXT refined analysis and potential detection DATE: 26/05/05 09:32:52 GMT FROM: SVOM_group <svomgroup@bao.ac.cn> P. Maggi (ObAS), D. Götz (CEA), H. Goto (RIKEN/CEA), M. Moita (CEA), C. Plasse (UHK), F. Robinet (IJCLab), C. Van Hove (IJCLab) report of behalf of the SVOM/MXT Team: GRB 260504B (Saccardi et al. GCN 44454) was observed by SVOM/MXT after an automatic SVOM slew, starting at T_MXT = 2026-05-04T09:33:16, 117 s after trigger time T0. MXT observed for the remainder of the orbit for 730s and the subsequent 5 orbits. Using the full X-band dataset, an uncatalogued X-ray source is only detected significantly at the very start of the MXT observation. Restricting the analysis until T_MXT+60s, the position of the MXT candidate afterglow is refined to: R.A. (J2000) = 23h24m10.8s Dec (J2000) = -80d17m01s with a 90% C.L. radius of 113” (including 25 arcseconds systematic error added in quadrature). This position is compatible with the SVOM/ECLAIRS position, but not with the SVOM/MXT position using the onboard processed data, initially reported. This position supersedes the one published in the GCN 44454. The light curve exhibits a fast decay with temporal index alpha < -6 (with count rate proportional to t^alpha). At t >T_MXT + 60s the source is no longer detected by MXT. The source is below the MXT detection limit in subsequent orbits. We analysed the spectrum extracted from the first 60s of observation, modelled with an absorbed power-law. The absorbing column was fixed to the Galactic NH of 7x1e20 /cm². A photon index Gamma = 1.3 +/- 0.4 is measured. We measure a 0.3-8 keV flux of 2.6 (-1.6/ +2.28) x 1e-10 erg/s/cm² (all uncertainties at 90% C. L.; these preliminary results may be improved) The Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA), French Space Agency (CNES), and the Chinese Academy of Sciences (CAS), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. MXT was developed jointly by CEA, CNES, University of Leicester, IJCLab and MPE. The Burst Advocate (BA) on shift for this alert is Yun Wang: wangyun@pmo.ac.cn. Please contact the BA by email if you require additional information. View this GCN Circular online at https://gcn.nasa.gov/circulars/44468. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...