GCN Circular 13229
Subject
GRB120404A: GROND observations show steeply decaying afterglow
Date
2012-04-05T15:30:10Z (13 years ago)
From
Arne Rau at MPE <arau@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.