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
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
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.