GCN Circular 9874

F. Olivares, P. Afonso, J. Greiner (all MPE Garching), S. McBreen (MPE + 
UCD), T. Kruehler, A. Rau, A. Yoldas, G. Kanbach (all MPE), and S. Klose 
(TLS Tautenburg) report on behalf of the GROND team:

The field of GRB 090902B, (Fermi GBM trigger 273582310/090902462, Bissaldi 
et al., GCN #9866) which was detected by the LAT (de Palma et al., GCN 
#9867), was observed simultaneously in g'r'i'z'JHK with GROND (Greiner et 
al. 2008, PASP 120, 405) mounted at the 2.2 m ESO/MPI telescope at LaSilla 
Observatory (Chile).

Observations started at 00:21 UT on September 3, 2009, 13.25 hours after 
the GRB trigger. They were performed at a seeing of around 2", at an 
average air mass larger than 2.0 and under variable sky conditions. We 
detect a NIR source within the Swift/XRT error circle reported by Kennea et 
al. (GCN #9868) and on the edge of the Swift/XRT enhanced position (Evans 
et al. GCN #9871), at:

RA (J2000.0) = 17h 39m 45.41s
DEC (J2000.0) = +27d 19' 27.1"

with an uncertainty of 0.5" in each coordinate.

This position seems consistent with the object reported by Perley et al. 
(GCN #9870) and is very likely the source reported by Swenson & Siegel (GCN 
#9869).

The source is not in the field of view of the griz channels, so no 
statement can be made about the optical bands.

The afterglow (Cucchiara et al. GCN #9873) has the following preliminary AB 
magnitude and shows tentative evidence of fading within the observations.

midtime         J_AB mag
  -----------------------------------------------
01:09:31 UT     20.26 +/- 0.07

Calibrations was done against 2MASS catalog stars. No Galactic extinction 
correction was applied.