TITLE:   GCN CIRCULAR
NUMBER:  29653
SUBJECT: GRB 210312B: GTC photometry
DATE:    21/03/13 00:51:28 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 R. Scarpa (GTC) report:

We observed the afterglow (Jelinek et al., GCN #29651; Lipunov et al., 
GCN #29652) of INTEGRAL GRB 210312B (Mereghetti et al., GCN #29650) with 
OSIRIS at the 10.4m Gran Telescopio Canarias (Roque de los Muchachos 
Observatory, La Palma, Canary Islands, Spain), starting at 2021-03-12 
22:57:10.834 UT. We obtained a 60 s finding chart exposure under adverse 
conditions (2" seeing, bad transparency because of Calima).

In the image, the afterglow is clearly detected. Against a nearby 
PanSTARRS star, we measure r' = 21.52 +/- 0.06 mag at 2.0898 hr after 
the trigger (0.08708 days).