GCN Circular 30982

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.