M. Tanga (MPE Garching), F. Olivares E. (UNAB, MCSS), and J. Greiner
(MPE Garching) report on behalf of the GROND team:
We observed the field of GRB 130725B (Swift trigger 564028; Swenson et al.,
GCN #15029) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La
Silla Observatory (Chile).
Observations started at 00:47 UT on July 26, 2013, 7.1 hours after the
GRB trigger. They were performed at an average seeing of 1.5" and at
an average airmass of 1.2.
We detect the afterglow at the position of Siegel et al. (GCN #15032), and
based on the first images with total exposures of 4.4 minutes in g'r'i'z'
and 8 minutes in JHK, we estimate preliminary magnitudes (all in AB) of
g' = 21.1 +/- 0.1 mag,
r' = 20.5 +/- 0.1 mag,
i' = 20.4 +/- 0.1 mag,
z' = 20.4 +/- 0.1 mag,
J = 20.4 +/- 0.3 mag,
H = 19.9 +/- 0.3 mag, and
K = 19.5 +/- 0.4 mag.
Given magnitudes are calibrated against GROND zeropoints as well as
2MASS field stars and are not corrected for the expected Galactic
foreground extinction corresponding to a reddening of E(B-V) = 0.061 mag in
the direction of the burst (Schlegel et al. 1998).