TITLE: GCN CIRCULAR NUMBER: 30982 SUBJECT: GRB 211023A: OAJ Bright Afterglow Confirmation DATE: 21/10/25 00:45:45 GMT FROM: Alexander Kann at IAA-CSIC <kann@iaa.es> D. A. Kann (HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/DARK/NBI), C. C. Thoene, M. Blazek, J. F. Agui Fernandez (all HETH/IAA-CSIC), Iglesias-Marzoa, R., and Lacruz, E. (both OAJ) report: We observed the Fermi-LAT position (Di Lalla et al., GCN #30961) of the bright Fermi GRB 211023A (Fermi detection: Fermi GBM Team, GCN #30958; Lesage et al., GCN #30965; AGILE detection: Ursi et al., GCN #30969) with the JAST/T80 telescope of OAJ, Teruel, Spain. We obtained 3 x 300 s images each in g', r', and i' under good conditions. At the position of the MASTER afterglow candidate (Zhirkov et al., GCN #30977), we detect a bright source which is not visible in Pan-STARRS archival imaging. Against three Pan-STARRS comparison stars, we measure r' = 18.03 +/- 0.02 mag (AB mag) at 0.5704 d after the GRB. We note this is an exceedingly bright afterglow, just slightly fainter at similar times than those of GRBs 130427A and 181201A. We also note a resemblance in prompt emission shape, fluence (S. Poolakkil, priv. comm.), and peak energy to the famous GRB 030329. Spectroscopic observations and further follow-up are encouraged.