TITLE:   GCN CIRCULAR
NUMBER:  13335
SUBJECT: GRB 120521A: GROND Upper limits
DATE:    12/05/22 17:11:33 GMT
FROM:    Andrea Rossi at TLS Tautenburg  <rossi@tls-tautenburg.de>

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).