A. Rossi (Tautenburg), F. Olivares, T. Kruehler and J. Greiner (all
MPE Garching) report on behalf of the GROND team:
We observed the field of GRB 090531B (Swift trigger 353728, Cummings et al.
2009, GCN #9461) simultaneously in g'r'i'z'JHK with GROND
(Greiner et al. 2008, PASP 120, 405) mounted at the 2.2m MPI/ESO telescope
at La Silla Observatory (Chile).
Observations started on 1 June 2009 at 00:03 UT, 5.5 hours after the burst and
continued until 00:50 UT under mediocre seeing (~1.5), cirrus and Moon light.
A second epoch started at 04:33 UT under better conditions, consisting of
72 min exposure in g'r'i'z' and 60 min in JHK.
We find three objects within the XRT error circle (Sbaruffatti et al. 2009,
GCN #9463), and another three at the outer circumference of the error circle
(see finding chart on http://www.mpe.mpg.de/~jcg/grb090531B.html). Within
+-0.2 mag, none of these objects shows any variability between 5.5 and 10 hrs
post-burst, and thus are most likely foreground objects.
In empty regions of the error circle, the first epoch yields the following
upper limits (all AB):
g' > 22.9
r' > 22.6
i' > 22.1
z' > 22.8
J > 20.1
H > 19.8
K > 19.2
which were obtained using the USNO and 2MASS field stars as reference.
However, given the poor conditions and preliminary photometric calibration,
a variable object might be hidden behind one of the foreground objects,
and masked in the difference image due to the distorted PSF from the first
epoch. In that case, the upper limits would be about 2 mag brighter.
No correction has been applied for the strong foreground reddening of
E(B-V)=1.11 mag (Schlegel et al. 1998).