GCN Circular 17757

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
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 ) 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).