TITLE: GCN CIRCULAR NUMBER: 23486 SUBJECT: GRB 181201A: GROND detection of the afterglow and redshift upper limit of z < 3 DATE: 18/12/02 15:19:53 GMT FROM: Jan Bolmer at MPE/Garching 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.