GCN Circular 28376

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 V. Casanova (IAA-CSIC) report:

We observed the location (Beardmore et al., GCN #28373) of the Swift GRB 
200906A (Page et al., GCN #28371; GBM detection: Fermi GBM Team, GCN 
#28370, Hamburg et al., GCN #28374) with the 1.5m telescope of the 
Observatorio Sierra Nevada, in Granada, Spain. We obtained 78 x 100 s 
images beginning 2020-09-06 21:29:03.51 UT, after clouds had cleared.

In a stack of all images, centered at 0.39467 days after the GRB, no 
source is detected at the enhanced XRT position. Measured against two 
nearby USNO-B1.0 stars, we derive an upper limit of

Ic > 23.4 mag.

This agrees with the non-detection reported by Fu et al. (GCN #28372). 
We note the refined XRT analysis (Beardmore et al., GCN #28375) also 
reports an NH column density significantly exceeding the Galactic value, 
indicating this is a dark GRB.