GCN Circular 18461

F. Knust (MPE Garching), S. Klose (TLS Tautenburg), and J. Greiner 
(MPE Garching) report on behalf of the GROND team: 

We observed the field of GRB 151023A (Swift trigger 661253; D'Avanzo
et al., GCN #18450
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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 MPG telescope at ESO
La Silla Observatory (Chile). 
Observations started at 23:52 UT on Oct. 23rd 2015, 10 hrs after the
GRB trigger, and are continuing. They were performed at an average
seeing of 1.6" and at an average airmass of 1.6  
We found a single point source within the 2.3" Swift-XRT error circle
reported by Evans et al. (GCN #18459
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 ) at 

RA (J2000.0) = 18:03:56.244 
Dec. (J2000.0) = -08:18:57.42

with an uncertainty of 0.2" in each coordinate. 
Based on the first 4.4 min of total exposures in g'r'i'z' and 4.0 min
in JHK, we estimate preliminary magnitudes (all in AB system) of 

g'> 22.1 mag,
r' = 20.2 +/- 0.1 mag,
i' = 19.1 +/- 0.1 mag,
z' =18.4 +/- 0.1 mag,
J = 17.6 +/- 0.2 mag,
H = 17.0 +/- 0.1 mag, and
K = 17.3 +/- 0.2 mag.

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)=1.38
mag in the direction of the burst (Schlegel et al. 1998).         		 	   		  

[GCN OPS NOTE(24oct15):  The suffix "A" was added to the Subject-line.]