GCN Circular 19058

J. Bolmer (MPE Garching), D. A. Kann (TLS Tautenburg), T. Kruehler,
and J. Greiner (both MPE Garching) report on behalf of the GROND team:

We observed the field of GRB 160223A (Swift trigger 675361; Markwardt et
al., GCN #19057) simultaneously in g'r'i'z'JHK with GROND (Greiner
et al. 2008, PASP 120, 405) mounted at the 2.2m MPG telescope at the
ESO La Silla Observatory (Chile).

Observations started at 03:30:41 UT on 2016-02-23, 106 min after the GRB
trigger. They were performed at an average seeing of 1".2 and at an
average airmass of 1.3.

We find a bright, uncatalogued point source within the Swift-BAT error
circle at

RA (J2000.0) = 09:50:23.34
Dec. (J2000.0) = +09:22:09.0

with an uncertainty of 0".5 in each coordinate.

Based on the first 4.4 min of total exposure in g'r'i'z' and 4.0 min
in JHK at a mid-time of 03:34:13 UT, we derive the following preliminary
magnitudes (all in AB system):

g = 17.8 +- 0.1 mag
r = 16.9 +- 0.1 mag
i = 16.5 +- 0.1 mag
z = 16.2 +- 0.1 mag
J = 16.1 +- 0.1 mag
H = 15.9 +- 0.1 mag
K = 15.8 +- 0.1 mag

The object fades by 0.4 mag until 04:55 UT on 2016-02-23, so we
propose this source to be the optical/NIR afterglow of GRB 160223A.

Given magnitudes are calibrated against SDSS as well as 2MASS
field stars and are not corrected for the expected Galactic foreground
extinction corresponding to a reddening of E_(B-V) = 0.03 mag in the
direction of the burst (Schlafly & Finkbeiner 2011).

[GCN OPS NOTE(23feb16):  Per author's request, DAK was added to the author list.]