GCN Circular 10299

R. Filgas, T. Kruehler and J. Greiner (all MPE Garching) report on 
behalf of the GROND team:

We observed the field of GRB 091230 (INTEGRAL trigger #5967, D. Gotz et
al., GCN #10298) simultaneously in g'r'i'z'JHK with GROND mounted at the 
2.2m ESO/MPI telescope at La Silla Observatory (Chile).

Observations started on 30 Dec. 2009 at 06:30:27 UT, 2.9 min after the
burst.

Image subtraction reveals a variable point source inside the INTEGRAL
error circle (D. Gotz et al., GCN #10298) at

RA (J2000.0) = 08:51:39.18
DEC (J2000.0) = -53:53:52.7

with an uncertainty of 0.5" in each coordinate. This is 56" from the
refined INTEGRAL position.

Preliminary photometry yields the following magnitudes in stacked images:

Filter  T_mid[s] Exp[s]  AB Mag  MagErr
---------------------------------------
g'      390       4x 66  21.63   0.07
r'      390       4x 66  20.76   0.04
i'      390       4x 66  20.45   0.05
z'      390       4x 66  20.22   0.05
J       403      24x 10  19.75   0.12
H       403      24x 10  19.58   0.14
K       403      24x 10  19.26   0.15


The quoted error is statistical only. There is an additional systematic
error in the absolute calibration using the GROND zeropoints and 2MASS
field stars which is expected to be in the 0.1 to 0.2 mag range.

The object fades by two magnitudes to r' = 22.8 � 0.1 until the end of
our optical observations two hours after the trigger - thus we propose
this object as the optical/NIR afterglow of GRB 091230.