TITLE:   GCN CIRCULAR
NUMBER:  13526
SUBJECT: GRB 120728A: GROND observations
DATE:    12/07/29 02:20:44 GMT
FROM:    Alexander Kann at TLS Tautenburg  <kann@tls-tautenburg.de>

D. A. Kann, S. Klose (both TLS Tautenburg), and J. Greiner (MPE Garching)
report on behalf of the GROND team:

We observed the field of GRB 120728A (Swift trigger 529021; Cummings et
al., GCN 13525) 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 23:05 UT on 2012-07-28, 41 minutes after the GRB
trigger. Observations were obtained in twilight, at high (2.5) airmass and
bad seeing (2".7 in the r' band).

Based on an exposure of 9.4 minutes in the optical and 16 minutes in the
NIR we do not detect any source within the Swift/XRT error circle at
coordinates RA, Dec(J2000)= 09:08:22.69, -54:26:17.7, error 3".6. (SPER
data)

At the edge of the error circle, we find two sources. One, at RA (J2000)
= 09:08:23.20, Dec. (J2000) = -54:26:18.8, is a star also visible in the
DSS. The other may be marginally detected in the DSS, and likely consists
of two sources, centered at (+/- 0".5) RA (J2000) = 09:08:22.82, Dec.
(J2000) = -54:26:13.1 and RA (J2000) = 09:08:22.55, Dec. (J2000) =
-54:26:14.2. We are unable to make any statements about the variability
of these objects at this time, but given the crowded field this close to
the Galactic plane, it is likely neither is associated with the GRB.

Preliminary limiting magnitudes (AB mags) are:

  g' > 21.6
  r' > 21.9
  i' > 21.3
  z' > 21.1
  J  > 19.5
  H  > 19.9
  Ks > 19.3.

Magnitudes are derived based on GROND zeropoints (g'r'i'z') and 2MASS
stars (JHKs). No correction for the high Galactic reddening along the
line of sight (E(B-V)=0.57 mag; Schlegel et al. 1998) has been applied.