GCN Circular 10683

F. Olivares, T. Kruehler and J. Greiner (all MPE Garching) report on 
behalf of the GROND team:

We observed the field of GRB 100425A (Swift trigger 420398; Grupe et 
al., GCN 10673
Not found
 
 
) 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 05:17 UT on April 25, 2.4 hours after the GRB 
trigger, and were initially performed through thin cloud coverage.

The optical afterglow candidate reported by Malesani et al. (GCN 10680
Not found
 
 
, 
source 'B') is found to be clearly variable. Between 2.9 and 5.9 hours 
after the burst, the source decayed roughly 0.5 mag in the r' band. At a 
midtime of 08:45 UT we measure the following preliminary magnitudes (all 
in the AB system) in stacked images with a total integration time of 24 
min in g'r'i'z' and 20 min in JHK:

g = 21.5 +- 0.2
r = 21.0 +- 0.1
i = 20.8 +- 0.1
z = 20.5 +- 0.1
J = 20.1 +- 0.2
H = 19.9 +- 0.2
K = 19.6 +- 0.2

calibrated against the GROND zeropoints and 2MASS field stars, and not 
corrected for the expected foreground extinction of E(B-V)=0.15 
(Schlegel et al. 1998).