F. Knust, K. Varela, J. Greiner (MPE Garching), and D. A. Kann
(TLS Tautenburg) report on behalf of the GROND team:
We observed the field of GRB 140428A (Swift trigger 597519; Kocevski et
al., GCN #16177) 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 La Silla
Observations started at 00:43 UT on 2014-04-29, 2 hours after the GRB
trigger. They were performed at an average seeing of 1.4" and at an
average airmass of 1.9.
We found a single point source within the 1.7" Swift-XRT error circle
reported by Page et al. (GCN #16178) at
RA (J2000.0) = 12h 57m 28.39s
DEC (J2000.0) = + 28d 23' 06.4"
with an uncertainty of 0.5" in each coordinate.
This source is consistent with the one reported by Perley (GCN #16180).
Based on the first 1133 s of total exposures in g'r'i'z' and 1920 s in
JHK, we estimate preliminary magnitudes (all in AB system) of
g' > 24.4 mag,
r' = 23.6 +/- 0.2 mag,
i' = 21.7 +/- 0.1 mag,
z' = 21.5 +/- 0.1 mag,
J > 19.7 mag,
H > 20.6 mag, and
K > 19.1mag.
From our data, fading can not be established. We note that our r'-band
magnitude is similar to that reported by Perley (GCN #16180) at a mid-time
of 7.5 hrs after the GRB.
The spectral energy distribution is best-fit by a straight power-law and
a sharp cut-off below
the i'-band which cannot be modelled with extinction alone. If this is the
GRB afterglow, the cut-off corresponds to a photometric redshift of z =
4.8 +-0.3, consistent with the Keck spectroscopy
as reported by Perley (GCN #16181).
Given magnitudes are calibrated against SDSS (griz) as well as 2MASS
(JHK) field stars and are not corrected for the expected Galactic
foreground extinction corresponding to a reddening of E_(B-V) = 0.01 mag
in the direction of the burst (Schlegel et al. 1998).