A. Rossi (Tautenburg), F. Olivares, and J. Greiner (both MPE Garching)
report on behalf of the GROND team:
We observed the field of GRB 090530 (Swift trigger 353567, Cannizzo et al.,
GCN #9438) simultaneously in g'r'i'z'JHK with GROND mounted at the 2.2m
MPI/ESO telescope at La Silla Observatory (Chile). Observations started on
31 May 2009 at 00:36 UT, 21.3 h after the burst.
We detect the optical/NIR afterglow (Cannizzo et al., GCN #9438;
Flewelling et al., GCN #9439; Morgan et al., GCN #9440; Nissinen & Hentunen,
GCN #9442) in stacked exposures of 20 min in JHK and 24 min in g'r'i'z'.
Calibrating the r' band data against the USNO star at RA = 11:57:41.7,
Dec = +26:35:43, we obtain R = (22.2 +-0.2) mag. Comparing this with
the white light magnitudes obtained by ROTSE (Flewelling et al., GCN #9439),
UVOT (Schady et al., GCN #9450) and GRAS002 (Nissinen & Hentunen,
GCN #9442) as well as the NOT R-band magnitude (Malesani et al., GCN #9452),
we find a break in the optical light at around 22 ksec post-burst (i.e.
before the NOT observation), with slopes of 0.5+-0.1 (pre-break) and
1.8+-0.4 (post-break). We note that a break at this time is also visible
in the Swift/XRT data (see repository; Evans et al. 2007).