TITLE: GCN CIRCULAR NUMBER: 10572 SUBJECT: GRB 100331B: GROND Upper limits DATE: 10/04/03 10:40:30 GMT FROM: Jochen Greiner at MPI Adria Updike (Clemson University), Paulo Afonso, and Jochen Greiner (both MPE Garching) report on behalf of the GROND team: We observed the field of GRB 100331B (SuperAGILE; Del Monte et al., GCN #10560) 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 09:31 UT on April 1, 2010, 12.37 hours after the GRB trigger, and a second epoch was taken beginning at 08:50 UT on April 2. They were performed at an average seeing of 1.0" and at an average airmass of 1.3. We do not detect a source within the Swift-XRT error circle reported by D'Elia et al. (GCN #10562). Upper limits from the first epoch are limited to the NIR bands; the GROND observation was executed prior to the Swift/XRT observation, and the XRT position turned out to be outside the FOV of our optical channels. On April 1, our upper limits were: J > 21.5, H > 21.0, K > 20.2 from a total of 960 seconds of exposure time. On April 2, our upper limits were: g' > 24.6, r' > 25.1, i' > 24.5, z' > 24.4, J > 22.6, H > 22.1 and K > 20.9 from a total of 2952 seconds of exposure in the optical bands and 2320 seconds in the NIR. These limits are derived based on calibrating the images against GROND zeropoints and 2MASS field stars and are not corrected for the Galactic foreground extinction corresponding to a reddening of E_(B-V)= 0.12 in the direction of the burst (Schlegel et al. 1998).