A. Rossi (INAF-OAS) reports on behalf of the CIBO collaboration:
We observed the location of the optical afterglow (Malesani et al., GCN
29117) of the short GRB 201221D (Page et al., GCN 29112) simultaneously
in the J and Ks bands with the LUCI near-infrared imager and
spectrograph mounted on the Large Binocular Telescope (LBT, Mt Graham,
AZ, USA). Observations were obtained on 2020-12-24 at the UT midtime
11:40:00, i.e. ~2.5 days after the burst trigger, for a total of 20 min
of exposure in each band.
Inspection of the combined J and K-band images reveals a faint, extended
source in both filters, for which we preliminary measure J=21.7+-0.3
(Vega system), calibrated against 2MASS field stars.
We acknowledge the excellent support from the LBTO and LBT-INAF staff,
particularly A. Cardwell, F. Cusano, and D. Paris, in obtaining these