[AAVSO-HEN] GRB 090831C: GROND upper limits

[grb at aavso.org]

The following is a GCN Circular circulated via the AAVSO network.
For an archive and more information on circulars visit:
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TITLE:   GCN CIRCULAR
NUMBER:  9862
SUBJECT: GRB 090831C: GROND upper limits
DATE:    09/09/01 18:45:16 GMT
FROM:    Thomas Kruehler at MPE/MPI  <kruehler at mpe.mpg.de>

F. Olivares, T. Kruehler and J. Greiner (all MPE Garching) report on behalf 
of the GROND team:

We observed the field of GRB 090831C (Swift trigger #361489; Schady et al., 
GCN #9854) 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 08:07 UT on September 1st, 2009, 10.6 hr after the 
burst. At this time the position of the burst was becoming visible above 
the pointing constraints of the telescope. Observations suffer from high 
airmass and large seeing of around 2". In stacked images of 74 min total 
integration time in griz and 60 min in JHK, we do not detect any object 
within the enhanced XRT error circle (Evans et al., GCN #9855), down to the 
following limiting magnitudes (all in the AB system):

g' > 23.5
r' > 24.0
i' > 23.3
z' > 22.9
J > 21.4
H > 21.1
K > 20.0

These upper limits were derived using the GROND zeropoints and 2MASS field 
stars as reference and are not corrected for significant Galactic 
foreground extinction corresponding to a reddening of E(B-V)=0.38 mag 
(Schlegel et al., 1998).