TITLE:   GCN CIRCULAR
NUMBER:  9352
SUBJECT: GRB 090510: GROND observations
DATE:    09/05/12 15:19:57 GMT
FROM:    Thomas Kruehler at MPE/MPI  <kruehler@mpe.mpg.de>

F. Olivares (MPE Garching), S. Klose (Tautenburg), T. Kruehler, J. Greiner 
(both MPE Garching) report on behalf of the GROND team:

GROND, mounted at the 2.2m on La Silla, observed the field of GRB 090510, 
detected by Swift (Hoversten et al., GCN 9331), Fermi GBM (Guiriec et al., 
GCN 9336) and LAT (Ohno & Pelassa, GCN #9334), AGILE (Longo et al., GCN 
9343) and Konus-Wind (Golenetskii et al., GCN 9344), in g'r'i'z'JHK 
starting at 6:34 UT on 10 May 2009.

We detect the afterglow reported by Marshall & Hoversten (GCN #9332) and 
Olofsson et al. (GCN #9338) in the optical bands. At a midtime of 9.0 h 
after the burst, we measure the following AB magnitudes of the afterglow 
and upper limits in stacked images

g' = 23.5 +/- 0.3
r' = 23.0 +/- 0.1
i' = 22.8 +/- 0.1
z' = 22.6 +/- 0.1
J > 21.8
H > 21.4
K > 20.7

calibrated against the GROND zeropoints and 2MASS field stars. We caution 
that the photometry might be contaminated by the nearby source mentioned by 
Olofsson et al. (GCN #9338).

Together with the magnitude reported by NOT (Olofsson et al. GCN #9338), 
this implies a rapid early fading with an index of ~2 during this epoch, 
which is compatible to the X-ray afterglow at a similar time interval 
(Grupe & Hoversten, GCN #9341). A late flattening of the optical light 
curve is then indicated by the UVOT detection (Kuin et al. #GCN 9351).