TITLE:   GCN CIRCULAR
NUMBER:  16996
SUBJECT: GRB 141031A: GROND detection of a possible optical afterglow candidate
DATE:    14/10/31 11:35:31 GMT
FROM:    Corentin Delvaux at MPE  <delvaux@mpe.mpg.de>

M. Tanga, C. Delvaux (both MPE Garching), D. A. Kann (TLS Tautenburg), P.
Schady and J. Greiner (both MPE Garching) report on behalf of the GROND
team:

We observed the field of GRB 141031A (Swift trigger 617110; Gompertz et
al., GCN #16995) 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 the ESO La
Silla Observatory (Chile).

Observations started at 07:46 UT on 31 October 2014, 0.45 hrs after the
GRB trigger. They were performed at an average seeing of 1.3" and at an
average airmass of 1.33.

We found one single point source at the NW edge of the 2.1" Swift-XRT
error circle reported by Gompertz et al. (GCN #16995) at

RA (J2000.0) = 08h 34m 25.75s

DEC (J2000.0) = -59d 10' 05.0"

with an uncertainty of 0.3" in each coordinate.

Based on the first 8.8 min of total exposures in g'r'i'z' and 8.0 min in
JHK, we estimate preliminary magnitudes (all in AB system) of

g' = 22.7 +/- 0.1 mag,
r' = 21.7 +/- 0.1 mag,
i' = 21.5 +/- 0.1 mag,
z' = 21.5 +/- 0.1 mag,
J > 20.9 mag,
H > 20.3 mag, and
K > 18.7 mag.

At the moment we cannot confirm if the source is fading.

Given magnitudes are calibrated against GROND zeropoints as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V)=0.17 mag in the
direction of the burst (Schlegel et al. 1998).