TITLE:   GCN CIRCULAR
NUMBER:  26560
SUBJECT: GRB 191221B: LCO Optical Detection
DATE:    19/12/22 18:20:35 GMT
FROM:    Robert Strausbaugh at U. of the Virgin Islands  <robert.strausbaugh@uvi.edu>

R. Strausbaugh (U. of the Virgin Islands), A. Cucchiara (U. of the Virgin Islands/College of Marin) report on behalf of a larger collaboration:

We observed Swift GRB 191221B (Laha et al., GCN 26534) with the LCO 1-m Sinistro instrument at the Sutherland South African Astronomical Observatory site on December 22, from 00:30 to 01:11 UT (corresponding to 3.5 to 4.2 hours after the GRB trigger time) with the Bessel R and I filters.

We performed a series of 10x90s exposures in each band. We detect a bright source in the Swift-BAT refined error region (Laha et al., GCN 26536) that is not present in either USNO-B1.0 or 2MASS  surveys with the following  magnitudes:

R = 16.50 +/- 0.01

I = 15.86 +/- 0.01

This flux measurement is calibrated against several USNO-B1.0 objects near the GRB location and is not corrected for Galactic Extinction.