TITLE: GCN CIRCULAR NUMBER: 9874 SUBJECT: GRB 090902B: GROND NIR Afterglow Observations DATE: 09/09/03 09:14:50 GMT FROM: Thomas Kruehler at MPE/MPI 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.