GCN Circular 30834

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

We observed GRB 210912A (Swift detection: Troja et al., GCN #30820) with 
CAFOS mounted on the 2.2m CAHA telescope at Calar Alto, Almeria, Spain. 
We obtained 7 x 60 s in the SDSS i' band under passing clouds, starting 
at 20:04:34 UT (9.056 hrs after the trigger; further observations at a 
later time were prevented by cloud cover). The afterglow (Hu et al., GCN 
#30823; Pankov et al., GCN #30825; Kuin & Troja, GCN #30832) is faintly 
detected in the stacked image.

Compared to a nearby Pan-STARRS comparison star, we measure (AB mag) i' 
= 21.66 +/- 0.15 mag at 0.38146 d after the GRB, further confirming the 
decaying afterglow. This is still significantly brighter than the 
potential underlying host galaxy (Kann, GCN #30829).