TITLE:   GCN CIRCULAR
NUMBER:  13729
SUBJECT: GRB 120909A: GROND detection of the afterglow
DATE:    12/09/09 02:41:37 GMT
FROM:    Vladimir Sudilovsky at MPE  <vsudilov@mpe.mpg.de>

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

We observed the field of GRB 120909A (Swift trigger 533060; Immler et al.,
GCN #13727) simultaneously in g'r'i'z'JHK 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 01:44 UT, September 09, 140s after the burst, and
are continuing. They were performed at an average seeing of 1".6 and at an
average airmass of 1.2 .

We found a single point source within the 4".8 Swift-XRT error circle
reported by Immler et al. (GCN #13727) at

RA (J2000.0) = 18h 22m 56.72s

DEC (J2000.0) = -59d 26' 54".1

with an uncertainty of 0".5 in each coordinate, which is consistent with
the position reported by Haislip et al. (GCN #13728)   .

Based on the first 141s of total exposures in g'r'i'z' and 240s in JHK,
we estimate preliminary magnitudes (all in AB) of

g' = 18.4 +- 0.1,
r' = 16.6 +- 0.1,
i' = 15.8 +- 0.1,
z' = 15.5 +- 0.1,
J = 14.8 +- 0.1,
H = 14.5 +- 0.1, and
K = 14.3 +- 0.1.

Based on the the significantly lower flux in g', this suggests a 
redshift z>3.

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=0.08 mag in the
direction of the burst (Schlegel et al. 1998).