TITLE:   GCN CIRCULAR
NUMBER:  29319
SUBJECT: GRB 210112A: Deep CAHA 2.2m detection
DATE:    21/01/18 15:53:55 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 J. I. Vico Linares (CAHA) report:

We observed the afterglow position (Ambrosi et al., GCN #29289) of the 
bright Swift GRB 210112A (Swift detection: Ambrosi et al., GCN #29289; 
AGILE detection: Ursi et al., GCN #29293; Konus-Wind detection: Svinkin 
et al., GCN #29315) with CAFOS at the 2.2m telescope at Calar Alto, 
Almeria, Spain, in the Ic band. We obtained 10 x 360 s exposures, 
centered at 5.1696 days after the GRB, under good conditions but 
mediocre seeing.

The afterglow (Kann et al., GCNs #29296,29300; Siegel et al., GCN 
#29303) is faintly detected. We measure Ic = 23.69 +/- 0.24 mag. (AB 
mag, vs. a several nearby SDSS stars converted to Ic following the 
Lupton transformations, then transformed back to AB mag).

This magnitude lies slightly above the extrapolation of the decay found 
during the first two epochs (Kann et al., GCN #29300) and may indicate 
an underlying host galaxy. Indeed we also see another faint, possibly 
extended source about 2" to the East, at RA (J2000) = 14:36:01.453, Dec. 
(J2000) = +33:03:13.40, which may be the host galaxy or a system 
interacting with it. However, the rough redshift estimate of z ~ 2 (Kann 
et al., GCN #29296) would imply this must be a very luminous system, or 
a foreground interloper.