TITLE: GCN CIRCULAR NUMBER: 9458 SUBJECT: GRB 090530: break in light curve DATE: 09/05/31 12:36:48 GMT FROM: Jochen Greiner at MPI 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).