TITLE: GCN CIRCULAR NUMBER: 13229 SUBJECT: GRB120404A: GROND observations show steeply decaying afterglow DATE: 12/04/05 15:30:10 GMT FROM: Arne Rau at MPE V. Sudilovsky, A. Rau , J. Greiner (all MPE Garching), report on behalf of the GROND team: We observed the field of GRB120404A (Swift trigger 519380; Stratta et al., GCN #13208) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla Observatory (Chile). Observations started at 2012-04-05 07:01:16 UT, 18.2 hours after the GRB trigger. The afterglow (e.g., Stratta et al., GCN #13208; Guidorzi et al., GCN #13209) is clearly detected. At mid-time of 19.3 hours post trigger and with an exposure of 20 min we measure the following preliminary SDSS (g'r'i'z') and 2MASS (JHK) calibrated AB magnitudes: g' = 23.2 ± 0.3 r' = 22.6 ± 0.1 i' = 22.3 ± 0.1 z' = 22.0 ± 0.1 J = 21.2 ± 0.3 H = 20.6 ± 0.3 K > 20.0 The SED, corrected for the Galactic foreground reddening of E_(B-V)=0.05 mag (Schlegel et al. 1998) is best fit by a power law with a slope of beta~1.4 and no additional rest frame extinction. The comparison with the MITSuME observation at 4.8 hours post-trigger (Kuroda et al., GCN #13224) and TNT measurement at 7.4 hours post-trigger (Xin et al., GCN #13221) suggests a significant steepening of the afterglow decay. Assuming the decay started around ~7.4 hours post-trigger, we estimate the decay slope to be ~2.2.