GCN Circular 28227

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 L. Montoya (CAHA) report:

We observed the afterglow position (Beardmore et al., GCN #28214; 
Marshall et al., GCN #28222) of the bright Swift GRB 200806A (Ambrosi et 
al., GCN #28211; Lien et al., GCN #28223) with CAFOS at the 2.2m 
telescope at Calar Alto, Almeria, Spain, in the SDSS i' band. We 
obtained 20 x 180 s exposures, centered at 0.49185 days after the GRB, 
under good conditions but moonlight.

At the position of the potential UVOT afterglow (Marshall et al., GCN 
#28222), we detect a faint source not visible in PanSTARRS archival 
imaging, at position (J2000) RA = 03:31:43.76, Dec. = +37:04:43.90, with 
an estimated error of 0".75.

Photometry is made complicated by an uneven background, but measured 
against two nearby SDSS stars, we derive i' = 22.95 +/- 0.25 mag.

As this is significantly fainter than the UVOT detection, we consider 
this to be the afterglow of GRB 200806A. We note foreground extinction 
is rather high (E(B-V)=0.3564 mag according to the maps of Schlafly & 
Finkbeiner 2011), and there is additional hydrogen column density along 
the line-of-sight (Page et al., GCN #28221), implying this is likely a 
dark GRB.