We have systematically studied the spectral properties of 302 localized gamma-ray bursts (GRBs) observed by the Suzaku wide-band all-sky monitor (WAM) from 2005 August to 2010 December. The energy spectra in the 100-5000keV range integrated over the entire emission and the 1s peak were fitted by three models: a single power law, a power law with an exponential cutoff (CPL), and the GRB Band function (GRB). Most of the burst spectra were well fitted by a single power law. The average photon index alpha was -2.11 and -1.73 for long and short bursts, respectively. For the CPL and GRB models, the low-energy and high-energy photon indices (alpha and beta) for the entire emission spectra were consistent with previous measurements. The averages of the alpha and beta were -0.90 and -2.65 for long-duration GRBs, while the average alpha was -0.55 and the beta was not well constrained for short-duration GRBs. However, the average peak energy E-peak was 645 and 1286keV for long- and short-duration GRBs respectively, which are higher than previous Fermi/GBM measurements (285keV and 736keV). The alpha and E-peak of the 1s peak spectra were larger, i.e., the spectra were harder, than the total fluence spectra. Spectral simulations based on Fermi-GBM results suggest that the higher E(peak)s measured by the Suzaku WAM could be due to detector selection bias, mainly caused by the limited energy range above 100keV.

We will review the 10 years history of all-sky observations in the hard X-ray to soft gamma-ray energy band by the Suzaku Wide-band All-sky Monitor. The Suzaku-WAM is the shield detector of the Hard X-ray detector onboard Suzaku and the primary purpose is to guard the main detector and the background rejection. In addition, the large effective area of these shield detectors ($\sim$ 600 cm$^2$ at 1 MeV and the large field of view ($>$ 2 $\pi$ str) enable us to perform all-sky observations in 50 $-$ 5000 keV. During 10 years operations, the WAM detected more than 1400 GRBs and many other transients such as soft gamma repeaters and solar flares. This detection rate of GRBs is comparable to that of other GRB-specific missions. Thanks to its large effective area, a time-resolved properties of these astronomical transients up to the MeV energy band were investigated with good photon statistics.

We will review results for gamma-ray bursts (GRBs) and soft gamma repeaters (SGRs), obtained from the Suzaku Wide-band All-sky Monitor (WAM) which operated for about 10 years from 2005 to 2015. The WAM is a BGO (bismuth germanate: Bi4Ge3O12) lateral shield for the Hard X-ray Detector (HXD),used mainly for rejecting its detector background, but it also works as an all-sky monitor for soft gamma-ray transients in the 50-5000 keV range thanks to its large effective area (similar to 600 cm(2) at 1MeV for one detector) and wide field of view (about half of the entire sky). The WAM actually detected more than 1400 GRBs and 300 bursts from SGRs, and this detection number is comparable to that of other GRB-specific instruments. Based on the 10 years of operation, we describe timing and spectral performance for short GRBs, weak GRBs with high redshifts, and time-resolved pulses with good statistics.

The detection of a gamma-ray burst (GRB) in the solar neighborhood would have very important implications for GRB phenomenology. The leading theories for cosmological GRBs would not be able to explain such events. The final bursts of evaporating Primordial Black Holes (PBHs), however, would be a natural explanation for local GRBs. We present a novel technique that can constrain the distance to gamma-ray bursts using detections from widely separated, non-imaging spacecraft. This method can determine the actual distance to the burst if it is local. We applied this method to constrain distances to a sample of 36 short duration GRBs detected by the Interplanetary Network (IPN) that show observational properties that are expected from PBH evaporations. These bursts have minimum possible distances in the 10^13-10^18 cm (7-10^5 AU) range, consistent with the expected PBH energetics and with a possible origin in the solar neighborhood, although none of the bursts can be unambiguously demonstrated to be local. Assuming these bursts are real PBH events, we estimate lower limits on the PBH burst evaporation rate in the solar neighborhood.

We report on the T-90 and T-50 duration distributions and their relations with spectral hardness using 1464 gamma-ray bursts (GRBs), which were observed by the Suzaku Wide-band All-sky Monitor (WAM) from 2005 August 4 to 2010 December 29. The duration distribution is clearly bimodal in three energy ranges (50-120, 120-250, and 250-550 keV), but is unclear in the 550-5000 keV range, probably because of the limited sample size. The WAM durations decrease with energy according to a power-law index of -0.058(-0.034, +0.033). The hardness-duration relation reveals the presence of short-hard and long-soft GRBs. The short: long event ratio tends to be higher with increasing energy. We compared the WAM distribution with ones measured by eight other GRB instruments. The WAM T-90 distribution is very similar to those of INTEGRAL/SPI-ACS and Granat/PHEBUS, and least likely to match the Swift/BAT distribution. The WAM short: long event ratio (0.25:0.75) is much different from Swift/BAT (0.08:0.92), but is almost the same as CGRO/BATSE (0.25:0.75). To explain this difference for BAT, we examined three effects: BAT trigger types, energy dependence of the duration, and detection sensitivity differences between BAT and WAM. As a result, we found that the ratio difference could be explained mainly by energy dependence including soft extended emissions for short GRBs and much better sensitivity for BAT which can detect weak/long GRBs. The reason for the same short: long event ratio for BATSE and WAM was confirmed by calculation using the trigger efficiency curve.