TITLE: GCN CIRCULAR NUMBER: 13048 SUBJECT: GROND observations of GRB 120311A DATE: 12/03/12 15:19:16 GMT FROM: Vladimir Sudilovsky at MPE V. Sudilovsky, T. Prinz, and J. Greiner (all MPE Garching) report on behalf of the GROND team: We observed the field of GRB 120311A (Markwardt et al., GCN #13032) 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 centered on the BAT position started on March 11, 2012, at 08:02 UT, 2.5h after the GRB trigger. They were performed at an average seeing of 1.0", and average airmass of 1.8. Based on a 20 minute exposure centered at 09:09 UT, the afterglow candidate reported by Melandri et al. (GCN 13033) is measured as follows (all in the AB system and calibrated against GROND zeropoints (g'r'i'z') and 2MASS (JHK)): g' = 22.9 +/- 0.3 r' = 22.5 +/- 0.1 i' = 22.4 +/- 0.1 z' = 22.0 +/- 0.1 J > 20.5 H > 19.6 K > 18.0 These above magnitudes have not been corrected for the Galactic extinction of E(B-V) = 0.13 towards the direction of the burst (Schlegel et al. 1998). These magnitudes imply a fading of more than two magnitudes relative to the observations reported by Melandri et al. (GCN 13033), and thus confirm the afterglow nature of this object.