TITLE:   GCN CIRCULAR
NUMBER:  29725
SUBJECT: GRB 210321A: OSN Upper Limit
DATE:    21/03/24 23:41:30 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann@iaa.es>

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

We observed the optical afterglow (Perri et al. GCN #29678; Siegel et 
al. GCN #29679; Butler et al. GCN #29680; Heintz et al. GCN #29684; Xin 
et al., GCN #29687; Pankov et al., GCNs #29689, #29692; Laporte et al., 
GCN #29697) of GRB 210321A (Swift detection: Dichiara et al., GCN 
#29677) with the 1.5m T150 telescope of the Observatorio de Sierra 
Nevada (OSN), Granada, Spain. Observations could not be taken due to 
high winds in the first night after the GRB. We obtained 11 x 180 s 
exposures in Rc starting at 2021-03-22 19:59:44.93 UT, before high winds 
shut the telescope down again. The afterglow is not detected in the 
combined image.

We determine an upper limit Rc > 23.3 mag (AB mag, vs. a nearby 
Pan-STARRS star converted to Rc following the Lupton transformations, 
then transformed back to AB mag) at 1.5437 days after the GRB.