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GCN Circular 15309

GRB 131004A: GROND observations of the afterglow
2013-10-05T08:38:02Z (11 years ago)
Sebastian Schmidl at TLS Tautenburg <>
S. Schmidl, A. Nicuesa Guelbenzu, S. Klose (all TLS Tautenburg), and J.
Greiner (MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 131004A (Swift trigger 573190; Hagen et al.,
GCN 15303) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008,
PASP 120, 405) mounted at the 2.2m MPG telescope at ESO La Silla
Observatory (Chile).

Observations started on October 4, 2013, at 23:41 UT, 2 hrs after the GRB
trigger. They were performed at an average seeing of 0".9 and at an
average airmass of 1.1.

The optical afterglow, discovered by Xu et al. (GCN 15304), is clearly

Based on a total exposure time of 460 s in g'r'i'z' and 480 s in JHK, at a
midtime of 2.75 hrs after the burst, we measure the following preliminary
AB magnitudes:

g' = 22.9 +/- 0.1,
r' = 22.1 +/- 0.1,
i' = 21.6 +/- 0.1,
z' = 21.4 +/- 0.1,
J = 20.2 +/- 0.2,
H = 20.2 +/- 0.2, and
K > 19.8 .

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)=0.29 mag
in the direction of the burst (Schlegel et al. 1998).
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