{
  "format": "text/plain",
  "createdOn": 1754457116935,
  "eventId": "GRB 250805A",
  "circularId": 41242,
  "subject": "GRB 250805A: SVOM/VT optical counterpart",
  "submitter": "Huali Li at at NAOC, SVOM <lhl@nao.cas.cn>",
  "body": "H. L. Li, Y. N. Ma, L. P. Xin, Y. L. Qiu, C. Wu, Z. H. Yao, Y. Xu, J. Wang, P. P. Zhang, W. J. Xie, Y. J. Xiao, H. B. Cai, L. Lan, J. S. Deng, J. Y. Wei (NAOC), J. Palmerio (CEA), A. Li (BNU), Y. D. Hu (GXU) report on behalf of the SVOM/VT team.\n\nSVOM performed a Target of Opportunity observation of GRB 250805A detected by Swift/BAT (Ambrosi et al., GCN 41233; Evans et al., GCN 41234; Page et al., GCN 41240). SVOM/VT began observing the field at 2025-08-05 07:00:42 UTC, 50.4 min after the Swift trigger time, in the VT_B (400nm-650nm) and VT_R (650nm-1000nm) channels simultaneously.\n\nAn uncatalogued source, compared to Lagacy survey, is found using VT X-band data, with a distance of 2.7 arcsec to Swift/XRT enhanced position (Beardmore et al., GCN 41238). The optical source is at R.A., Dec 24.216108, -81.402177 degrees:\n \n  RA  (J2000) =  01:36:51.86\n  Dec (J2000) = -81:24:07.84\nwith an uncertainty of 0.5 arcsec.\n \nThe source is detected in both VT_B and VT_R images. It decayed by 0.7 mag  within 1.89 hours of VT observation. The magnitudes are derived as below:\n \nMid time            | exposure time (s) | band | mag (AB) | mag err\n--------------------|-------------------|------|----------|--------\n1.24 hours          | 37*70             | VT_B | 22.79    | 0.10   \n1.24 hours          | 37*70             | VT_R | 21.16    | 0.04    \n\nOur photometry was not corrected for Galactic extinction.\n\nGiven the fading of its brightness, the uncatalogued source is proposed to be the optical afterglow of the burst. Our result is consistent with the report by ESO/VLT (Izzo et al., GCN 41241). \n\nThe color of VT_B-VT_R~1.5 mag suggests that it shall be a low or intermediate redshift GRB.\n\nThe Space Variable Objects Monitor (SVOM) is a China-France joint mission led by the Chinese National Space Administration (CNSA, China), National Center for Space Studies (CNES, France) and the Chinese Academy of Sciences (CAS, China), which is dedicated to observing gamma-ray bursts and other transient phenomena in the energetic universe. VT was jointly developed by Xi'an Institute of Optics and Precision Mechanics (XIOPM), CAS and National astronomical observatories (NAOC), CAS.",
  "submittedHow": "web",
  "bibcode": "2025GCN.41242....1L"
}