GCN Circular 23486

J. Bolmer (MPE, Garching) and P. Schady (Univ. of Bath) report:

We observed the field of GRB 181201A (INTEGRAL trigger; Mereghetti et al., GCN #23469)
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted
at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile).

Observations started at 00:16 UT, 21.85 hrs after the GRB trigger, and were performed
under good seeing conditions, 1.03'', and at an average airmass of 1.43.

We detect a source at a position consistent with the X-ray and optical afterglow
(Page et al., GCN #23474; Podesta et al., GCN #23470; Heintz et al., GCN #23478).
Based on the first 17 min of total exposures in g'r'i'z' and 20 min in JHK, we
measure the following preliminary AB magnitudes:

g' = 18.14 +/- 0.03 mag
r' = 17.97 +/- 0.03 mag
i' = 17.80 +/- 0.03 mag
z' = 17.64 +/- 0.03 mag
J = 17.55 +/- 0.05 mag
H = 17.28 +/- 0.05 mag
K = 17.13 +/- 0.08 mag

Given magnitudes are calibrated against SDSS and 2MASS field stars and
are not corrected for the expected Galactic foreground extinction corresponding
to a reddening of E_(B-V)=0.05 in the direction of the burst (Schlafly &
Finkbeiner 2011).

From a fit to the SED with a simple powerlaw including SMC-like dust
extinction and a model for absoprtion from the Lyman-alpha forest we
determine an upper limit for the redshift of z < 3 and  A_V < 0.10 mag.
The powerlaw index is -0.50.