TITLE: GCN CIRCULAR NUMBER: 17757 SUBJECT: GRB 150424A: GROND detects strong fading DATE: 15/04/24 23:55:22 GMT FROM: Alexander Kann at TLS Tautenburg D. A. Kann (TLS Tautenburg), M. Tanga, and J. Greiner (both MPE Garching) report on behalf of the GROND team: We observed the field of the extremely bright short GRB 150424A (Swift trigger 638946; Beardmore et al., GCN #17743) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPG telescope at ESO La Silla Observatory (Chile). Observations started at 23:09 UT on 2015-04-24, 15.4 hrs after the GRB trigger. They were performed at an average seeing of 1".2 and at an average airmass of 1.1. We detect the optical afterglow (Perley & McConnell, GCN # 17745; Malesani et al., GCN # 17756). Based on 7.7 min of total exposure time in g'r'i'z' and 8.0 min in JHK, centered 15.54 hrs after the GRB, we estimate the following preliminary magnitudes (all in AB system): g' = 22.0 +/- 0.2 mag, r' = 21.6 +/- 0.1 mag, i' = 21.4 +/- 0.1 mag, z' = 21.1 +/- 0.1 mag, J = 20.4 +/- 0.5 mag, and H > 19.9 mag. Compared to the NOT detection about two hours earlier, the afterglow has faded by 0.7 magnitudes, indicating the transition to a very rapid decay. Given magnitudes are calibrated against GROND zero points (g'r'i'z') as well as 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.06 mag in the direction of the burst (Schlegel et al. 1998).