GCN Circular 17729

K. Varela, F. Knust, J. Bodensteiner and J. Greiner (all MPE Garching)
report on behalf of the GROND team:

We observed the field of GRB 150423A (Swift trigger 638808; Pagani et al.,
GCN #17728) 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 ESO La Silla
Observatory (Chile).

Observations started at 06:32 UT on 2015-04-23, 4 min after the GRB
trigger. They were performed at an average seeing of 0".8 and at an
average airmass of 1.5.

We detect a faint source within the 2.2" XRT error circle, at position

RA (2000.0) = 14h 46m 18.9s
Decl (2000.0) = +12d 17' 00"

with an error of 0.5", about 1" away from the XRT centroid position.

Based on 7.7 min of total exposure time in g'r'i'z' and 8.0 min in JHK, 
centered 24 min after the GRB, we estimate the following preliminary
magnitudes (all in AB system):

g' = 23.2 +/- 0.2 mag,
r' = 23.1 +/- 0.2 mag,
i' = 22.5 +/- 0.2 mag,
z' = 22.0 +/- 0.2 mag,
J  = 19.3 +/- 0.4 mag,
H  > 18.4  mag, and
K  > 17.5 mag.

No statement can be made about fading at this stage. Since there is a
faint source also in the SDSS, this might be the host galaxy rather than
the afterglow.

Given magnitudes are calibrated against SDSS (g'r'i'z') 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 (Schlegel et al. 1998).