TITLE:   GCN CIRCULAR
NUMBER:  13992
SUBJECT: GRB 121123A: GROND observations
DATE:    12/11/24 03:38:49 GMT
FROM:    Sylvio Klose at TLS Tautenburg  <klose@tls-tautenburg.de>

S. Schmidl, A. Nicuesa Guelbenzu, S. Klose (all TLS Tautenburg), and J.
Greiner (MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 121123A (Swift trigger 539358; Helder et al.,
GCN 13982; Fermi trigger 375357963 / 121123421; Foley et al., GCN 13985)
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120,
405) mounted at the 2.2m MPG/ESO telescope at La Silla Observatory
(Chile).

Observations started at 00:20 UT on Nov 24, about 15 hours after the GRB
trigger. They were performed at an average seeing of 1.3" and at an
average airmass of 1.9. The optical afterglow (Helder et al., GCN 13982;
Zhao et al., GCN 13983; Xu et al., GCN 13986; Guziy et al., GCN 13987) is
clearly detected in all bands. Based on 1500 s of total exposures in
g'r'i'z' and 1200 s in JHK, we measure the following preliminary AB
magnitudes at a midtime of 15.25 hrs after the burst:

g' = 20.8 +/- 0.1,
r' = 20.3 +/- 0.1,
i' = 19.9 +/- 0.1,
z' = 19.8 +/- 0.1,
J  = 19.2 +/- 0.1,
H  = 18.9 +/- 0.1,
K  = 18.6 +/- 0.2,

calibrated against GROND zeropoints and 2MASS stars. The SED can be fit
with a power law with a slope of beta = 1.2 +/ -0.1 and a g-band affected
by a dropout with a redshift of z = 2.7 +/- 0.3, taking into acount a
Galactic reddening along the line of sight of E_(B-V)= 0.05 mag (Schlegel
et al.1998). We note that the data can also be fit with a modest host
extinction and no dropout-affected g-band. At present we cannot solve this
ambiguity.