TITLE: GCN CIRCULAR NUMBER: 18624 SUBJECT: GRB 151120A: GROND optical/NIR afterglow DATE: 15/11/20 09:58:39 GMT FROM: Thomas Kruehler at MPE Garching 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) 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), 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).