[vsnet-grb-info 13933] GRB 130925A: GROND afterglow candidate

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  15247
SUBJECT: GRB 130925A: GROND afterglow candidate
DATE:    13/09/25 06:05:22 GMT
FROM:    Vladimir Sudilovsky at MPE  <vsudilov at mpe.mpg.de>

V. Sudilovsky (MPE Garching), D.A. Kann, and J. Greiner
(all MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 130925A (Lien et al., GCN #15246)
simultaneously in g'r'i'z'JHKs with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La Silla
Observatory (Chile).

The first epoch observation started at 04:18:52 UT on 25 Sept 2013,
7 minutes after the GRB trigger. They were performed at an average
seeing of 0.7" and at an average airmass of 1.3. We do not find any
sources within the revised 3".6 Swift XRT error circle. However, we do
find an uncataloged red source 2" east of the error circle at position:

RA  (J2000.0) = 02:44:42.96 = 41.1790
Dec. (J2000.0) = -26:09:11.16 = -26.1531

with an uncertainty of 0.3" in each coordinate.

Based on a total exposure of 4.4 minutes in g'r'i'z' and 4 minutes in
JHK, we estimate preliminary magnitudes (all in AB) of

g' > 21.5 mag,
r' > 22 mag,
i' = 21.8 ± 0.1 mag,
z' = 21.3 ± 0.1 mag,
J = 19.6 ± 0.1 mag,
H = 18.6 ± 0.1 mag, and
K = 18.0 ± 0.1 mag.

The source is seen to be brightening in the first images before
fading strongly. We suggest this source to be the afterglow of GRB
130925A.

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) = 0.02
mag in the direction of the burst (Schlegel et al. 1998).