TITLE:   GCN CIRCULAR
NUMBER:  33447
SUBJECT: GRB 230307A: Gemini-South Confirmation of the Optical Afterglow
DATE:    23/03/10 03:37:36 GMT
FROM:    Brendan O'Connor at UMD  <oconnorb@umd.edu>

B. O'Connor (UMD, GWU), S. Dichiara (PSU), E. Troja (UTV, ASU),
J. Gillanders (UTV), S. B. Cenko (NASA, UMD), C. Kouveliotou (GWU):

We observed GRB 230307A (GBM team GCN 33405, Xiong et al. GCN 33406,
Kozyrev et al. GCN 33425) with the GMOS-S instrument mounted on
Gemini-South in Cerro Pachon, Chile through Director's Time.
Observations began at  00:06:47 UT on 2023-03-10, corresponding to
2.35 d after the GRB trigger.

In a 30 s acquisition image, we detect the optical afterglow reported
by Levan et al. (GCN 33439) and Lipunov et al. (GCN 33441). With
preliminary photometry calibrated to the SkyMapper catalog we derive
a magnitude of r~22.0+/-0.3 mag. This confirms fading of the reported
counterpart and solidifies the connection to the GRB.

Analysis of the spectroscopic observations is underway.

We thank the staff of the Gemini Observatory for rapidly approving and
executing these observations.