GCN Circular 13048

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
Not found
 
 
) 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
Not found
 
 
), and thus confirm
the afterglow nature of this object.