TITLE:   GCN CIRCULAR
NUMBER:  15665
SUBJECT: GRB 140102A: GROND observations
DATE:    14/01/03 15:03:39 GMT
FROM:    Sylvio Klose at TLS Tautenburg  <klose@tls-tautenburg.de>

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

We observed the field of GRB 140102A (Hagen et al., GCN 15653) 
simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, PASP 120, 
405) mounted at the 2.2m MPG telescope at La Silla Observatory (Chile).

Observations started at 07:51 UT on Jan 03, about 10.5 hrs after the GRB 
trigger. The afterglow discovered by Hagen et al. (see also Gorosabel et 
al., GCN 15658; Xu et al., GCN 15655) is detected in all optical bands. At 
a mean time of 08:19 UT, we measure the following preliminary AB 
magnitudes:

g'= 21.6 +/- 0.1,
r'= 21.4 +/- 0.1,
i'= 20.9 +/- 0.1,
z'= 21.0 +/- 0.1,
J = 20.7 +/- 0.3,
H = 20.6 +/- 0.4,
K > 19.3,

calibrated against SDSS and 2MASS field stars.

After correcting for a Galactic reddening of E(B-V)=0.03 mag (Schlegel et 
al. 1998), the spectral slope defined by g'r'z'JH is ~ 0.7, with no 
evidence for host extinction.

The SED fit shows a slight excess in i'-band flux. It could be due to 
emission line(s) from the underlying host, though we cannot say anything 
about an underlying galaxy due to the bad image quality (FWHM ~ 2.5"). We 
note, however, the presence of a galaxy just 4" East of the afterglow.