V. Sudilovsky, M. Tanga, and J. Greiner
(all MPE Garching) report on behalf of the GROND team:
We observed the field of GRB 130831B (Swift trigger 568855;
Malesani et al. 2013, GCN #15141) simultaneously in g'r'i'z'JHK with
GROND (Greiner et al. 2008, PASP 120, 405) mounted at the
2.2 m MPG/ESO telescope at La Silla Observatory (Chile).
Observations started at 23:22 UT on 31 Aug. 2013, 10 hours after the
GRB trigger. They were performed at an average seeing of 2.2" and at
an average airmass of 2.2.
Based on five co-added images with individual exposures of 6 min in
g'r'i'z' and 4 min exposure in JHK, we do not detect a source within the
3.6 arcsec Swift-XRT error circle reported by Malesani et al.
(2013; GCN #15141) down to (all in the AB system)
g' > 23.4,
r' > 23.2,
i' > 22.6,
z' > 22.3,
J > 20.8,
H > 20.2, and
K > 19.6
The given limits are derived based on calibrating the images against
GROND zeropoints and 2MASS field stars and are not corrected for the
Galactic foreground extinction corresponding to a reddening of
E_(B-V)= 0.08 in the direction of the burst (Schlegel et al. 1998).