F. Knust (MPE Garching), D. A. Kann (TLS Tautenburg), T. Kruehler (ESO),
and J. Greiner (MPE Garching) report on behalf of the GROND team:
We observed the field of GRB 150428A (Swift trigger 639275; Page et al.,
GCN #17765) 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
Observations started at 02:24 UT on 2015-04-28, 53 minutes after the GRB
trigger, following a delay due to the labeling of the event as "not a GRB"
in the BACODINEs. They were performed at an average seeing of 0".9 and at
an average airmass of 1.2.
We detect a source at (J2000):
RA = 12:34:09.26
Dec. = +06:57:08.7
with an error of 0".5 in each coordinate.
Based on 25 min of total exposure time in g'r'i'z' centered 1.41 hrs after
the GRB, we estimate the following preliminary magnitudes (all in AB
g' = 24.1 +/- 0.4 mag,
r' = 23.0 +/- 0.1 mag,
i' = 22.7 +/- 0.1 mag, and
z' = 23.2 +/- 0.2 mag.
At this time, we cannot establish fading. At this position, we also detect
a source in the SDSS, which implies that this would be the host galaxy of
this GRB. We note the hydrogen column density as derived from X-rays
(Page et al., GCN #17765) is much higher than the (very low) Galactic NH,
implying this may be an optically dust-suppressed event.
Given magnitudes are calibrated against SDSS field stars and are not
corrected for the expected Galactic foreground extinction corresponding to
a reddening of E_(B-V)=0.02 mag in the direction of the burst (Schlegel et