TITLE:   GCN CIRCULAR
NUMBER:  11954
SUBJECT: GRB110420A: GROND detection of the Optical/NIR Afterglow Candidate
DATE:    11/04/21 14:18:16 GMT
FROM:    Jonny Elliott at MPE/GROND  <jonnyelliott@mpe.mpg.de>

P. Afonso, J. Elliott, and J. Greiner (MPE Garching) report on behalf of
the GROND team:

We observed the field of GRB 110420A (Swift trigger 451757; Mangano et
al., GCN #11941) simultaneously in g'r'i'z'JHK with GROND (Greiner et al.
2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla
Observatory (Chile).

Observations started at 09:34 UT on April 21, 22.5 hours after the GRB
trigger. They were performed at an average seeing of 1.8'' and at an
average airmass of 2.3.

We confirm the detection of the single point source reported by Oates et
al. (GCN #11948).

Based on the first 142-s exposures in r', we estimate a preliminary
magnitude (AB system) of

r' = 20.6 +/- 0.1 mag.

In combination with the early UVOT detection, this would imply a decay
slope of 0.85 between 700 s and 22.5 hours.

Given magnitudes are calibrated against GROND zeropoints and are not
corrected for the expected Galactic foreground extinction corresponding to
a reddening of E_(B-V)=0.014 mag in the direction of the burst (Schlegel
et al. 1998).