TITLE: GCN CIRCULAR NUMBER: 10676 SUBJECT: GRB 100424A: GROND Upper Limits DATE: 10/04/25 06:32:41 GMT FROM: Thomas Kruehler at MPE/MPI F. Olivares, T. Kruehler and J. Greiner (all MPE Garching) report on behalf of the GROND team: We observed the field of GRB 100424A (Swift trigger 420367; Hoversten et al., GCN 10667) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory (Chile). Observations started at 00:16 UT on April 25, 7.7 hours after the GRB trigger, and continued for 3 hours. The observations were severely affected by passing cirrus clouds. In our first stacked images of 24 min total integration time in g'r'i'z' and 20 min in JHK, we do not (see also Malesani et al., GCN 10671, Levan et al., GCN 10672, Morgan & Bloom GCN 10675) detect a source within the enhanced Swift/XRT error circle (Evans et al., GCN 10669) down to the following preliminary 3 sigma limiting magnitudes (all in the AB system): g' > 23.03 r' > 23.37 i' > 22.96 z' > 23.04 J > 21.77 H > 21.21 K > 20.29 Later images do not increase these limiting magnitudes due to worse sky conditions. These limits were derived by calibrating the images against SDSS/2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.03 in the direction of the burst (Schlegel et al. 1998).