GCN Circular 13335
A. Rossi (TLS Tautenburg), M. Nardini (Universita' degli Studi Milano-Bicocca), S. Klose (TLS Tautenburg), and J. Greiner (MPE Garching) report on behalf of the GROND team: We observed the field of GRB 120521A (Swift trigger 522578; Chester et al., GCN 13302) simultaneously in g'r'i'z'JHKs with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla Observatory (Chile). Observations started at 22:53 UT on 2012-05-21, 16.9 h after the GRB trigger, and lasted for 100 minutes. They were performed under good sky conditions at an average seeing of 0.9 and at an average airmass of 1.1. Using an exposure of 50 minutes in the optical and 40 minutes in the NIR we do not detect any source within the Swift-XRT error circle reported by Goad et al. (GCN 13304). Preliminary observed upper limits (AB mags) are: g' > 25.5 r' > 25.0 i' > 24.0 z' > 23.7 J > 20.9 H > 20.3 K > 19.1 The given limits are derived based on calibrating the images against GROND zeropoints (g'r'i'z') and 2MASS stars (JHK). We note that the Galactic reddening along the line of sight is E_(B-V)= 0.43 mag (Schlegel et al. 1998).