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GCN Circular 13729

GRB 120909A: GROND detection of the afterglow
2012-09-09T02:41:37Z (12 years ago)
Vladimir Sudilovsky at MPE <>
V. Sudilovsky (MPE Garching), S. Schmidl (TLS Tautenburg), D.A. Kann (MPE
Garching) and J. Greiner (MPE Garching) report on behalf of the GROND

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).
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