TITLE: GCN CIRCULAR NUMBER: 31000 SUBJECT: GRB 211023B: LDT Optical Afterglow Detection DATE: 21/10/25 23:11:12 GMT FROM: Brendan O'Connor at UMD <oconnorb@umd.edu> B. O'Connor (UMD, GWU),S.B. Cenko (UMD,NASA-GSFC), E. Troja (UMD, NASA-GSFC),A.Gottlieb (UMD,NASA-GSFC), S.Dichiara (PSU),A. Kutyrev (UMD,NASA-GSFC), S. Veilleux (UMD) We observed the field of GRB 211023B (Dichiara et al., GCN 30960) using the Large Monolithic Imager (LMI) on the 4.3m Lowell Discovery Telescope (LDT) at Happy Jack, AZ. Observations began on October 25, 2021 at 11:42:14 UT (~1.61 d after the GRB), and were performed in r-band for a total of 18x150 s exposures under poor conditions. We detect a source coincident with the position of the optical counterpart reported by UVOT (Kuin et al., GCN 30975) and CAHA (Kann et al., GCN 30992). We measure a brightness of r~23 AB mag, brighter than the archival object reported by Pozanenko et al. (GCN 30972). We do not detect any other source within 5" radius of the UVOT position with a 3-sigma upper limit r>24.0 mag. Given the non-detection at ~15 hours (Watson et al., GCN 30973), our detection implies a shallow temporal decay index beta<0.6, assuming a power-law decline as F~t^-beta. While this may be due in part to the underlying source (Pozanenko et al., GCN 30972), we note that the same flattening is also observed in X-rays by Swift/XRT. Therefore, we strongly encourage continued follow-up of this source. Magnitudes are calibrated against the SDSS catalog and are not corrected for Galactic extinction. We thank the staff of the Lowell Discovery Telescope for assistance with these observations.