TITLE: GCN CIRCULAR NUMBER: 13328 SUBJECT: GRB 120521B: GROND Upper limits DATE: 12/05/22 08:24:45 GMT FROM: Sylvio Klose at TLS Tautenburg 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 120521B (Swift trigger 522586; Cummings et al., GCN 13305) 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 00:35 UT on 2012-05-22, 15.3 h after the GRB trigger, and lasted for 140 minutes. They were performed with good sky conditions at an average seeing of 0.8" and at an average airmass of 1.05. (At the time of the GRB trigger the field was already too close to the telescope's horizon distance limit in order to observe it with GROND.) Based on an exposure of 37 minutes in the optical and 30 minutes in the NIR we do not detect any source within the Swift-XRT error circle reported by Osborne et al. (GCN 13311). Preliminary observed upper limits (AB mags) are: g' > 24.5 r' > 24.2 i' > 23.9 z' > 23.9 J > 21.3 H > 20.3 Ks > 19.3. These limits are derived based on 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)= 0.45 mag (Schlegel et al. 1998).