GCN Circular 11013
F. Olivares, R. Filgas and J. Greiner (all MPE Garching) report on behalf of the GROND team: We observed the field of GRB 100728B (Swift trigger 430172; Morris et al., GCN #11009) simultaneously in 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 10:34 UT, 2 minutes after the GRB trigger. They were performed during astronomical twilight at an average airmass of 1.2. We detect a single point source at position reported by Perley et al. (GCN #11007), Morris et al. (GCN #11009), Ivarsen et al. (GCN #11011) and Elenin et al. (GCN #11012). Based on the first 4 min of total exposures in JHK, we estimate preliminary magnitudes (all in AB system) of J = 16.4 +- 0.1 mag, H = 16.2 +- 0.1 mag and K = 16.0 +- 0.1 mag Given magnitudes are calibrated against 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.07 mag in the direction of the burst (Schlegel et al. 1998).