{
  "eventId": "GRB 260504B",
  "submittedHow": "web",
  "submitter": "SVOM_group <svomgroup@bao.ac.cn>",
  "subject": "GRB 260504B: SVOM/MXT refined analysis and potential detection",
  "createdOn": 1777973572404,
  "format": "text/plain",
  "body": "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:\n\nGRB 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.\n\nUsing 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:\nR.A. (J2000) = 23h24m10.8s \nDec (J2000) = -80d17m01s\nwith 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.\n\nThe 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. \nThe source is below the MXT detection limit in subsequent orbits.\n\nWe 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)\n\nThe 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.\n\nThe Burst Advocate (BA) on shift for this alert is Yun Wang: wangyun@pmo.ac.cn.\nPlease contact the BA by email if you require additional information.",
  "circularId": 44468
}