TITLE: GCN CIRCULAR NUMBER: 10683 SUBJECT: GRB 100425A: GROND afterglow observation DATE: 10/04/25 10:46:55 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 100425A (Swift trigger 420398; Grupe et al., GCN 10673) 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 05:17 UT on April 25, 2.4 hours after the GRB trigger, and were initially performed through thin cloud coverage. The optical afterglow candidate reported by Malesani et al. (GCN 10680, source 'B') is found to be clearly variable. Between 2.9 and 5.9 hours after the burst, the source decayed roughly 0.5 mag in the r' band. At a midtime of 08:45 UT we measure the following preliminary magnitudes (all in the AB system) in stacked images with a total integration time of 24 min in g'r'i'z' and 20 min in JHK: g = 21.5 +- 0.2 r = 21.0 +- 0.1 i = 20.8 +- 0.1 z = 20.5 +- 0.1 J = 20.1 +- 0.2 H = 19.9 +- 0.2 K = 19.6 +- 0.2 calibrated against the GROND zeropoints and 2MASS field stars, and not corrected for the expected foreground extinction of E(B-V)=0.15 (Schlegel et al. 1998).