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GCN Circular 9874

GRB 090902B: GROND NIR Afterglow Observations
2009-09-03T09:14:50Z (15 years ago)
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 

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.
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