TITLE: GCN CIRCULAR NUMBER: 26580 SUBJECT: GRB 191221B: ATCA detection of the radio afterglow DATE: 19/12/25 16:43:35 GMT FROM: Tanmoy Laskar at U of Bath T. Laskar (University of Bath), S. Bhandari (CSIRO), K. D. Alexander (Northwestern), E. Berger (Harvard), R. Chornock (Ohio U.), D. Coppejans (Northwestern), M. Drout (U. Toronto), H. van Eerten (University of Bath), Wen-fai Fong (Northwestern), C. Guidorzi (U. Ferrara), G. Hosseinzadeh (Harvard), R. Margutti (Northwestern), C. G. Mundell (University of Bath), and P. Schady (University of Bath) report: "We observed GRB 191221B (Laha et al., GCN 26534) with the Australia Telescope Compact Array (ATCA) at multiple frequencies and multiple epochs beginning on 2019 December 22 14:17 UT (0.73 days after the burst). We detect a varying radio counterpart at 5.5, 9.0, 16.7, and 21.2 GHz at a position consistent with the X-ray position (Beardmore et al., GCN 26540) and optical position (Lipunov et al., GCN 26537). We identify this source as the radio afterglow of GRB 191221B. Further analysis and observations are on-going. We thank the CSIRO staff for approving these observations."