The Wideband All-sky Monitor (WAM) onboard Suzaku is a very powerful Gamma-Ray Burst (GRBs) monitor, thanks to its very wide energy range of 50 keV-5 MeV and large effective area of 400 cm2 even at 1 MeV. GRB observations with the WAM enable us to study the high-energy tail of the distribution of the Epeak and to explore the hard emission tail around MeV energy region. The WAM has already detected 126 GRBs from 2005 August to 2006 August and some of them were also detected simultaneously with other GRB satellites. The GRB detection rate is about 120 per year, in agreement with that expected before launch. The duration distribution of GRBs obtained with the WAM is bimodal, which is consistent with the BATSE results. Here we report the preliminary results of the GRB spectral properties up to MeV energy region, such as the Epeak distribution, the Epeak-Eiso correlation, and the spectral evolution.
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