GCN Circular 16666

D. A. Kann (TLS Tautenburg), C. Delvaux, and J. Greiner (both MPE
Garching) report on behalf of the GROND team:

We observed the field of GRB 140801A (Fermi trigger 428612396; Pelassa,
GCN #16658) 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 09:43 UT on 2014-08-02, 0.61 days after the GRB
trigger. They were performed at an average seeing of 1".5 and at an
average airmass of 2.1.

Based on images with exposure times of 1584 s in g'r'i'z', we detect the
optical afterglow at the MASTER position reported by Gorbovskoy et al.
(GCN #16653) at the following AB magnitudes:

g' = 22.4 +/- 0.1 mag,
r' = 21.9 +/- 0.1 mag,
i' = 21.8 +/- 0.1 mag, and
z' = 21.2 +/- 0.2 mag.

Observations taken a day later show the afterglow to have decayed by about
0.75 magnitudes. The afterglow continues to decay with a slope of ~0.9 in
comparison to the magnitudes given by Xu et al. (GCN #16656) and Moskvitin
et al. (GCN #16663).

The given magnitudes are derived based on calibrating the images against
the GROND zeropoints and are not corrected for the Galactic foreground
extinction corresponding to a reddening of E_(B-V) = 0.23 in the direction
of the burst (Schlegel et al. 1998).