TITLE: GCN CIRCULAR NUMBER: 31038 SUBJECT: GRB 211023B: Subaru optical observation DATE: 21/11/04 10:42:09 GMT FROM: Takanori Sakamoto at AGU <tsakamoto@phys.aoyama.ac.jp> T. Sakamoto (AGU), G.S.H. Paek (SNU), Y. Urata (NCU), E. Troja (NASA/GSFC), M. Im (SNU), K. Aoki (NAOJ) report on behalf of Subaru GRB team: We observed the field of GRB 211023B (Dichiara et al., GCN Circ. 30960) with Hyper Suprime-Cam (HSC) on the Subaru 8.2 m telescope starting from 13:36 on October 28 about 4.7 days after the trigger. Observations were performed in the r filter for a total of 4800 s (32 x 150 s) exposure. At a position consistent with the UVOT localization (Kuin & Dichiara, GCN Circ. 30975) and the Legacy Survey object (Pozanenko et al. GCN 30972), we detect an optical source with magnitude r~23.6 AB. Since the source appears clearly extended in our images and its brightness is consistent with the Gemini-North observation (Rastinejad et al., GCN Circ. 31002), archival HSC g-band images of March 2016, and CFHT archival r-band images of September 2020, we conclude that the source detected by Subaru is dominated by the host galaxy contribution, in agreement with Rastinejad et al. (GCN Circ. 31002). We would like to thank the Subaru director, Dr. Michitoshi Yoshida for approving our ToO, and Dr. Tsuyoshi Terai and Dr. Ichi Tanaka for supporting our observation.