GCN Circular 13891

F. Knust, P. Schady and J. Greiner (all MPE Garching) report on behalf  
of the GROND team

We observed the field of GRB 121024A (Pagani et al., GCN 13886
Not found
 
 
)  
simultaneously in g'r'i'z' with GROND (Greiner et al. 2008, PASP, 120,  
405) mounted on the 2.2 m MPG/ESO telescope at La Silla Observatory  
(Chile).

Due to clouds, observations only started at 05:53 UT no 24th October,  
3 hours after the GRB trigger, and continued until the end of the  
night, 3.8 hours later. They were performed at an average seeing of  
1.4" and at an average airmass of 1.1.

We find a single point source within the 0.8 arcsec Swift/UVOT error  
circle reported by Pagani et al. (GCN 13886
Not found
 
 
) at

RA (J2000.0) = 04:41:53.30
Dec (J2000.0) = -12:17:26.5

with an uncertainty of 0.4" in each coordinate.

Based on the first 142 sec integration in g'r'i'z' and 240 sec  
integration in JHK taken at a mid-time of 05:58 UT and 05:54 UT in the  
optical and NIR respectively, we estimate preliminary magnitudes (all  
in AB system) of

g' = 21.0 +/- 0.2 mag
r' = 20.1 +/- 0.2 mag
i' = 19.8 +/- 0.2 mag
z' = 19.5 +/- 0.2 mag
J = 18.7 +/- 0.1 mag
H = 18.4 +/- 0.1 mag
K = 17.9 +/- 0.1 mag

Given magnitudes are calibrated against GROND zeropoints in the  
optical, and 2MASS field stars in the NIR, and are not corrected for  
the expected Galactic foreground extinction corresponding to a  
reddening of E(B-V) = 0.1 mag in the direction of the burst (Schlegel  
et al. 1998).