GCN Circular 10652

Robert Filgas, Jochen Greiner (both MPE) and Adria Updike (Clemson Univ.)
report on behalf of the GROND team:

We observed the field of GRB 100423A (Swift trigger 420247; Stroh et al., 
GCN 10651
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 ) simultaneously in g'r'i'z'JHK with GROND (Greiner et al. 2008, 
PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory 
(Chile). Observations started at 00:37 UT on April 23, 2010, 150 sec after the
Swift trigger. They were performed at an average seeing of 1" and at airmass
of 1.8, respectively.

We found a single variable point source within the 3' error circle, not
visible in either 2MASS or DSS2 at 

RA  (J2000.0) =  09h 05m 50.22s
Dec (J2000.0) = +21d 29' 54.3"

with an uncertainty of 0.3" in each coordinate. From the first 4 min
observation we estimate the following preliminary magnitudes 
g = 17.7+-0.1
r = 17.6+-0.1
i = 17.5+-0.1
z = 17.4+-0.1
J = 17.3+-0.1
H = 16.9+-0.1
K = 16.5+-0.1

calibrated against SDSS and 2MASS, and not corrected for the expected
foreground extinction of E(B-V)=0.04 (Schlegel et al. 1998).

The object faded by 0.3 mag between the first and second 4 min observation,
and thus is likely the GRB afterglow.

Since the object is seen in all GROND bands, the redshift is smaller than 
about 3.5. We encourage spectroscopic observations.