K. Varela (MPE Garching), D. A. Kann, S. Klose (both TLS Tautenburg),
and J. Greiner (MPE Garching) report on behalf of the GROND team:
We observed the field of the Fermi LAT-detected GRB 140928A (Fermi GBM
trigger 433592996; Desiante et al., GCN #16847) simultaneously in
g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the
2.2m MPG telescope at ESO La Silla Observatory (Chile).
Observations started at 08:43.51 UT on 2014-09-29, 22.23 hrs after the GRB
trigger. They were performed at an average seeing of 1".5 and at an
average airmass of 1.2.
We detect a source inside the enhanced 1".5 Swift-XRT error circle (Pagani
et al., GCN #16848), at
RA (J2000.0) = 02:54:47.72
DEC (J2000.0) = -55:55:44.0
with an uncertainty of 0".5 in each coordinate.
Based on total exposures of 460 s in g'r'i'z' and 480 s in JHK, we measure
the following preliminary magnitudes and upper limits (AB magnitude
g' = 21.3 +/- 0.1,
r' = 20.8 +/- 0.1,
i' = 20.7 +/- 0.1,
z' = 20.4 +/- 0.1,
J = 20.2 +/- 0.2,
H = 20.1 +/- 0.2, and
K > 18.8.
We propose this source to be the optical/NIR afterglow of GRB 140928A. At
present we cannot decide if the source is fading.
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) = 0.01 mag in the
direction of the burst (Schlegel et al. 1998).