TITLE:   GCN CIRCULAR
NUMBER:  30674
SUBJECT: GRB 210820A: LT optical upper limit
DATE:    21/08/21 17:30:40 GMT
FROM:    Martin Blazek at HETH/IAA-CSIC  <alf@iaa.es>

M. Blazek, D. A. Kann (both HETH/IAA-CSIC), A. de Ugarte Postigo (HETH/
IAA-CSIC, DARK/NBI), C. C. Thoene, J. F. Agui Fernandez (both HETH/IAA-CSIC)
report:

We observed the location of the Swift GRB 210820A (Beardmore et al., GCN
#30664) with 2-m Liverpool Telescope located in La Palma, Canary Isands,
Spain. We obtained 15 x 60 s images beginning 2021-08-21 03:29:30 UT. In
a stack of all images, centered at 0.4175 days after the GRB, no source is
detected at the UVOT position. Measured against three nearby SDSS stars, we
derive an upper limit of r' > 21.1 mag. This agrees with the non-detections
reported by Lipunov et al. (GCN #30666), Fu et al. (GCN #30669)., and Zhu et
al. (GCN #30670).