TITLE:   GCN CIRCULAR
NUMBER:  29600
SUBJECT: GRB 210306A: LCO Optical Afterglow Detection
DATE:    21/03/06 07:40:57 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 210306A (D'Elia et al., GCN 29597) with the LCO 1-m Sinistro instrument at the McDonald Observatory, Texas, USA site, on March 6, from 06:49 to 07:06 UT (corresponding to 2.93 to 3.22 hours from the GRB trigger time) with the Bessel R and I filters.

We performed a series of 4x120s exposures in R and I. We clearly detect an optical source in R and I band in stacked images at a location consistent the initial UVOT detection (D'Elia et al., GCN 29597), in agreement with other optical afterglow detections (Watson et al., GCN 29598; Romanov, GCN 29599).  Using the USNO-B.1 catalog as reference, we calculate the following magnitudes:

R = 19.00 +/- 0.02

I = 17.70 +/- 0.03

These magnitudes are not corrected for galactic extinction.  The fainter R magnitude could indicate a moderately high redshift.

R.S. is funded by NSF AST grant #1831682