GCN Circular 18624

T. Kruehler, J. Greiner, J. Ridl (all MPE) and A. Hempel (Universidad 
Andres Bello) report on behalf of a larger collaboration:

We observed the field of GRB 151120A (INTEGRAL trigger 7295; Mereghetti
et al., GCN #18623
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 ) simultaneously in grizJHK 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 November 20, 2015, 08:27 UTC, 4 minutes after
the GRB trigger. They were performed at an average seeing of 1.3" and
at an average airmass of 1.2.

Inside the INTEGRAL error circle (Mereghetti et al., GCN #18623
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 ), we 
detect an uncatalogued and variable source, which we consider the 
optical/NIR afterglow of GRB 151120A. The object is at:

RA (J2000): 10:28:57.94
Dec (J2000): -32:31:35.1

with an estimated uncertainty of 0.5" in each coordinate.

Based on 4.4 min of total exposures in griz and 4 min in JHK obtained
at a midtime of 15 minutes after the trigger, we derive preliminary
magnitudes of:

g = 20.0 +- 0.1
r = 19.4 +- 0.1
i = 19.0 +- 0.1
z = 18.8 +- 0.1
J = 18.2 +- 0.1
H = 18.0 +- 0.2
K = 18.0 +- 0.3

These measurements are in the AB system and calibrated against GROND
zero-points in griz as well as 2MASS field stars in JHK. They are not
corrected for the expected Galactic foreground extinction corresponding
to a reddening of E_(B-V)=0.07 mag in the direction of the burst
(Schlafly & Finkbeiner et al. 2011).