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 150911A (Swift trigger 655262; Starling et
al, GCN 18280) 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 12, 2015, at 05:59 UT, 11.3 hrs after
the GRB trigger. They were performed at an average seeing of 2.2" and at an
average airmass of 1.8.
We do not detect a source within the 1.7" Swift-XRT error circle
reported by Goad et al. (GCN 18283).
Based on total exposures of 33 minutes in g'r'i'z'and 43 minutes in
JHK, at a midtime of 12.1 hrs after the burst, we measure the following
preliminary upper limits (AB magnitude system):
g'> 24.1 mag,
r'> 23.8 mag,
i'> 23.2 mag,
z'> 22.8 mag,
J > 21.6 mag,
H > 21.0 mag, and
K > 19.9 mag.
Given upper limits 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) = 0.32 mag
in the direction of the burst (Schlegel et al. 1998).