[vsnet-grb-info 27353] GRB 210112A: LBT host detection and correction to GCN 29319 (CAHA observations)

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  29456
SUBJECT: GRB 210112A: LBT host detection and correction to GCN 29319 (CAHA observations)
DATE:    21/02/10 15:12:35 GMT
FROM:    Alexander Kann at IAA-CSIC  <kann at iaa.es>

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) images and found that the source 
detected by Kann et al., GCN 29319, 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.