K. Varela, J. Graham, J. Greiner and D.A. Kann (all MPE Garching) report
on behalf of the GROND team:
We observed the field of the GRB 131202A (Kocevski et al., GCN #15559)
simultaneously in g'r'i'z'JHKs with GROND (Greiner et al. 2008, PASP 120,
405) mounted at the 2.2 m MPG telescope at La Silla Observatory (Chile).
Observations started at 01:01 UT on 03 December 2013, 9.5 hours after the
GRB. They were performed at an average seeing of 1.4" and at an average
airmass of 1.5.
Based on an observation with a total integration time of 6920 s in
g'r'i'z' and 5760s in JHK, we detect a source inside the enhanced XRT
circle (Osborne et al., GCN #15564) in the i' and z' bands, with
i' = 24.5 +/- 0.3 mag,
z' = 23.6 +/- 0.2 mag,
and 3 sigma limiting magnitudes (all in the AB system) :
g'> 25.5 mag,
r'> 25.3 mag,
J > 22.0 mag,
H > 21.4 mag,
K > 19.9 mag.
Magnitudes are calibrated against GROND zeropoints (griz) and 2MASS stars
(JHK), and are not corrected for the Galactic foreground extinction
corresponding to a reddening of E_(B-V) = 0.03 mag in the direction of the
burst (Schlegel et al. 1998).
Performing a joint fit with contemporary Swift X-ray data, we find that
the detections and upper limits can be explained with a low redshift and
high extinction (up to Av ~ 4.2 for z = 0 and SMC dust), with low
extinction and a redshift z ~ 5, or combinations in between. The
near-negligible NH column density at z = 0 ((1.3+1.0-0.8) x 10^21 cm^-2,
Gompertz et al., GCN #15567) and assuming a non-extreme dust-to-gas ratio
point to a higher probability for the z ~ 5 scenario.
We note that in any case, the significant detection in i' and the i'- z'
color conservatively imply z < 6, in contrast to the prediction by Ruffini
et al. (GCN #15576).