GCN Circular 17041

J. Graham (MPE Garching), S. Schmidl (TLS Tautenburg), and J. Greiner (MPE
Garching) report on behalf of the GROND team:

We observed the field of GRB 141109A (Swift trigger 618024; P. D'Avanzo
et al., GCN 17037
Not found
 
 
) 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 06:51 UT on November 09, 2014, approximately 1
hour after the GRB trigger. They were performed at an average seeing of
1.7" and at an average airmass of 1.6.

We detect the optical/NIR aftergow reported by Covino et al. (GCN 17039
Not found
 
 
)
and Xu et al. (GCN 17040
Not found
 
 
).

Based on total exposures of 25.0 minutes in g'r'i'z' and 20.0 minutes in
JHK, at a midtime of 2.3 hrs after the burst, we measure the following
preliminary magnitudes (AB magnitude system):

g' = 20.7 +/- 0.1 mag,
r' = 19.7 +/- 0.1 mag,
i' = 19.4 +/- 0.1 mag,
z' = 19.2 +/- 0.1 mag,
J = 19.0 +/- 0.1 mag,
H = 18.8 +/- 0.1 mag, and
K = 18.5 +/- 0.1 mag.

The magnitudes are calibrated against SDSS and 2MASS field stars and are
not corrected for the Galactic foreground extinction corresponding to a
reddening of E_(B-V)= 0.04 in the direction of the burst (Schlegel et al.
1998).