TITLE: GCN CIRCULAR NUMBER: 31002 SUBJECT: GRB 211023B: Gemini-North Optical Observations DATE: 21/10/26 01:38:16 GMT FROM: Jillian Rastinejad at Northwestern Univ. <jillianrastinejad2024@u.northwestern.edu> J. Rastinejad, K. Paterson, W. Fong, C. Kilpatrick, A. Rouco Escorial (Northwestern), Antonino Cucchiara (College of Marin) report on behalf of a larger collaboration: We observed the location of GRB 211023B (Dichiara et al. GCNC 30960) over two nights with the Gemini Multi-Object Spectrograph (GMOS) mounted on Gemini-North under Program GN-2021B-Q-109. We obtained 11x60-sec and 20x60-sec imaging in i-band at a mid-time of 2021 October 25.63 UT (0.7484 days post-burst) and 2021October 24.62 UT (1.745 days post-burst), respectively, both at a median airmass of 1.9. At 1.745 days post-burst, we detect a source consistent with both the UVOT position and the previously reported Legacy Survey source (Kuin et al. GCN 30975, Pozanenko et al. GCN 30972). Calibrated to SDSS, we measure a brightness of i_AB = 23.1 +/- 0.1 mag at seeing < 0.8'' for this source. This is consistent with previously reported upper limits (Watson et al., GCN 30973, Hu et al. GCN 30976, Pozanenko et al. GCN 30972) and indicates significant fading from the i-band measurement reported by Kann et al. (GCN 30995) at 0.3091 days post-burst. At present, the source appears marginally extended, and it is very likely that the underlying host galaxy is contributing significantly to our measured flux (based on the expected i-band magnitude of the host from interpolating Legacy Survey DR9 photometry; Dey et al., 2019). Further observations are planned to monitor the variability of the source. We thank Jennifer Andrews and additional Gemini staff for the rapid planning and execution of these observations.