GCN Circular 29456

A. Rossi (INAF-OAS), on behalf of the CIBO collaboration, and D. A. Kann 
(HETH/IAA-CSIC) report:

We observed the field of GRB210112A (Ambrosi et al., GCN #29289; Ursi et 
al., GCN #29293; Svinkin et al., GCN #29315) simultaneously in the r' 
and z' bands with the LBC imager mounted on the Large Binocular 
Telescope (LBT, Mt. Graham, AZ, USA). We obtained 20 min of imaging on 
2021-02-05, 24.3 days after the burst trigger. Observations were 
performed under mediocre seeing conditions and reached a depth of r' ~ 
26 mag.

At the position of the afterglow (Ambrosi et al., GCN #29289), we 
clearly detect the host galaxy in both filters and we measure the 
following AB magnitudes:

r' = 23.96 +/- 0.15 mag,
z' = 23.38 +/- 0.15 mag.

calibrated against SDSS field stars.

Therefore, we confirm that the flattening observed by Kann et al. (GCN 
#29319) was due to the host component.

However, we do not see the faint object detected by Kann et al. at RA, 
Dec. = 14:36:01.453, +33:03:13.40 (J2000). We have investigated the CAHA 
and OSN (Kann et al., GCN 29296
Not found
 
 
) images and found that the source 
detected by Kann et al., GCN 29319
Not found
 
 
, which should have been "South" of 
the afterglow position, not "East", is the actual afterglow (with the 
fainter, northern extension possibly stemming from image distortion or a 
slight mismatch when co-adding frames), the offset stemming from a small 
arcsecond-scale astrometry mismatch. For the afterglow position, we 
measure Ic = 23.52 �� 0.19 mag (AB), replacing the value given in GCN 
#29319. We estimate the underlying host to be Ic ~ 23.7 mag (AB) from 
the LBT photometry.

Correlating the OSN image with the LBT image, we find an afterglow 
position of RA, Dec. = 14:36:01.492 +33:03:14.945 (J2000), with errors 
of 0".2 in each coordinate, calibrated against Gaia DR2, in good 
agreement with the UVOT position (Ambrosi et al., GCN #29289).

In the LBT image, we furthermore detect a second, fainter, but probably 
also extended source just 2" north-north-east of the host (at RA, Dec. 
=14:36:01.54 +33:03:18.7; J2000) with r' = 24.5 �� 0.2 mag. It is unclear 
if this likely galaxy is related to the host galaxy.

We acknowledge the excellent support from the LBTO and LBT-INAF staff, 
particularly B. Rothberg and D. Paris, in obtaining these observations.