TITLE:   GCN CIRCULAR
NUMBER:  29296
SUBJECT: GRB 210112A: OSN Reddened Afterglow Detection
DATE:    21/01/12 19:28:59 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 (HETH), M. Blazek, J. F. Agui Fernandez (both 
HETH/IAA-CSIC), and A. Sota (IAA-CSIC) report:

We observed the optical afterglow of GRB 210112A (Swfit detection: 
Ambrosi et al., GCN #29289; AGILE Detection: Ursi et al., GCN #29293) 
with the 1.5m telescope of the Observatorio de Sierra Nevada (OSN), 
Granada, Spain. We obtained 12 x 90 s exposures in B, Rc, Ic and 11 x 90 
s in V, between 2021-01-12 04:03:22.52 and 2021-01-12 05:21:34.99 UT. 
The afterglow is detected In the combined image of each band.

At 0.12587 days after the GRB, we measure Rc = 22.41 +/- 0.12 mag (AB 
mag, vs. a several nearby SDSS stars converted to Rc following the 
Lupton transformations, then transformed back to AB mag).

A preliminary analysis of all data reveals peculiar colors, the Rc - Ic 
color is large, whereas the colors between B, V, and Rc are quite flat. 
There is no source detected in either SDSS or PanSTARRS at the afterglow 
position, so this is probably not due to influence of the host galaxy. 
However, we are able to fit this SED with a large (A_V > 1 mag) amount 
of Milky-Way like dust at a redshift of z ~ 2 and an intrinsic slope in 
agreement with the one derived from X-ray observations. Such high 
extinction would be in line with the significant excess column density 
seen in the XRT observations (Evans et al., GCN #29295).