GCN Circular 15080

K. Varela (MPE Garching), A.C. Updike (Roger Williams University),
J. Elliott (MPE Garching), J. Greiner (MPE Garching) and
D.A. Kann (TLS Tautenburg) report on behalf of the GROND team:

We observed the field of GRB 130623A (Cummings et al., GCN #14921)
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 09:15 UT on 25 June 2013,
33.7 hrs after the GRB trigger. They were performed at an average
seeing of 1."5 and at an average airmass of 1.6. We find a single
point source at the edge of the 4."5 Swift-XRT error circle reported
by Grupe et al. (GCN #14926) at

RA  (J2000.0) = 01:23:26.17 = 20.859063
DEC (J2000.0) = -77:46:24.0 = -77.773361

with an uncertainty of 0.3" in each coordinate.

Based on a total exposure of 50 minutes in g'r'i'z' and 40 minutes in JHK,
we estimate preliminary magnitudes (all in AB) of

g' = 22.5 +/- 0.1 mag,
r' = 22.4 +/- 0.1 mag,
i' = 22.2 +/- 0.1 mag,
z' = 21.9 +/- 0.1 mag,
J = 21.3 +/- 0.3 mag,
H = 20.5 +/- 0.3 mag, and
K > 20.2 mag.

The second epoch observations started at 08:29 UT on 05 August 2013,
42 days after the GRB trigger. They were performed at an average seeing of
1."3 and at an average airmass of 1.5. The source found in the first epoch
is not detected anymore: Based on a total exposure of 75 minutes in g'r'i'z'
and 60 minutes in JHK, we estimate preliminary upper limits (all in AB) of

g'  > 25.7
r'  > 25.2
i'  > 24.3
z'  > 24.0
J  > 21.8
H  > 20.9
Ks > 20.4

The fading by nearly 3 mag in the r'-band  proves it to have been the
afterglow of GRB 130623A. The clear detection down to the g'-band
implies a redshift smaller than 3.

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)=0.05 mag in the
direction of the burst (Schlegel et al. 1998).