[vsnet-grb-info 11838] GRB120404A: GROND observations show steeply decaying afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  13229
SUBJECT: GRB120404A: GROND observations show steeply decaying afterglow
DATE:    12/04/05 15:30:10 GMT
FROM:    Arne Rau at MPE  <arau at mpe.mpg.de>

V. Sudilovsky, A. Rau , J. Greiner (all MPE Garching), report on behalf 
of the GROND team:

We observed the field of GRB120404A (Swift trigger 519380; Stratta et 
al., GCN #13208) simultaneously in g'r'i'z'JHK with GROND (Greiner et 
al. 2008, PASP 120, 405) mounted at the 2.2 m MPG/ESO telescope at La 
Silla Observatory (Chile). Observations started at 2012-04-05 07:01:16 
UT, 18.2 hours after the GRB trigger.

The afterglow (e.g., Stratta et al., GCN #13208; Guidorzi et al., GCN 
#13209) is clearly detected. At mid-time of 19.3 hours post trigger and 
with an exposure of 20 min we measure the following preliminary SDSS 
(g'r'i'z') and 2MASS (JHK) calibrated AB magnitudes:

g' = 23.2 ± 0.3
r' = 22.6 ± 0.1
i' = 22.3 ± 0.1
z' = 22.0 ± 0.1
J = 21.2 ± 0.3
H = 20.6 ± 0.3
K > 20.0

The SED, corrected for the Galactic foreground reddening of E_(B-V)=0.05 
mag (Schlegel et al. 1998) is best fit by a power law with a slope of 
beta~1.4 and no additional rest frame extinction.

The comparison with the MITSuME observation at 4.8 hours post-trigger 
(Kuroda et al., GCN #13224) and TNT measurement at 7.4 hours 
post-trigger (Xin et al., GCN #13221) suggests a significant steepening 
of the afterglow decay. Assuming the decay started around ~7.4 hours 
post-trigger, we estimate the decay slope to be ~2.2.