S. Schmidl (TLS Tautenburg), F. Knust, and J. Greiner (both MPE Garching)
report on behalf of the GROND team:
We observed the field of GRB 150910A (SWIFT trigger 655097; Pagani et al,
GCN 18264) 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
Observations started on September 11, 2015, at 05:08 UT, 20.1 hrs after the
GRB trigger. They were performed at an average seeing of 2.8" and at an
average airmass of 2.2.
We clearly detect the optical afterglow reported by Pagani et al. (GCN
18264), Zheng & Filippenko (GCN 18265), Dichiara et al. (GCN 18266),
Kuroda et al. (GCN 18267), Xu et al. (GCN 18269), de Ugarte Postigo et al.
(GCN 18274) and Moskvitin et al (GCN 18275).
Based on total exposures of 7.7 minutes in g'r'i'z'and 4.0 minutes in
JHK, at a midtime of 20.8 hrs after the burst, we measure the following
preliminary magnitudes (AB magnitude system):
g' = 21.5 +/- 0.2 mag,
r' = 21.2 +/- 0.2 mag,
i' = 21.0 +/- 0.2 mag,
z' = 20.8 +/- 0.2 mag,
J > 19.8 mag,
H > 19.3 mag, and
K > 18.1 mag.
Given magnitudes and upper limits are calibrated against SDSS as well as
2MASS field stars and are not corrected for the expected Galactic
foreground extinction corresponding to a reddening of E_(B-V) = 0.05 mag
in the direction of the burst (Schlegel et al. 1998).