We report on the T90 and T50 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 T90 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.

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 (∼600 cm2 at 1 MeV 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.

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The energy dependence in light curves of gamma-ray bursts (GRBs) is a probe to study the underlying radiation mechanism. In particular, spectral evolution in the decay phase is expected to reflect the cooling process of accelerated electrons. Norris et al. [1] systematically examined asymmetric pulses in the prompt emissions of GRBs and showed that the pulse widths have the energy dependence that is well approximated with a power-law with the energy index of −0.41 in average. Although they did not particularly mention about the decay phase, their result strongly suggests a universal radiation/cooling mechanism in the emission region of GRBs. In previous study with Suzaku/WAM[2] , we sampled 6 bright GRBs; 7 well isolated pulses that showed no power-law decay but exhibiting exponential-decay (FRED) were detected in total, and found that the time constants evaluated for each energy band exhibited a power-law type energy dependence with the energy index of −0.3 to −0.5. Now our next step would be to investigate the radiation process in the GRB prompt emissions in wider energy bands beyond Suzaku/WAM. Here we report the results of our study of the three bright GRBs (GRB 060117, GRB 070917, GRB 080413B) that showed the FRED and were observed with both Swift/BAT and Suzaku/WAM. All of their exponential decays exhibit similar power-law type energy dependence. The distribution of the energy indices is consistent with the FREDs that were observed with WAM, as reported in [2]. Our detailed time-resolved spectral study reveals that the spectra of all the three FREDs are well reproduced with the Band GRB functions with decreasing turnover energy. In particular, the time evolution of two of the three FREDs are consistent with those expected in the fast synchrotron-cooling regime [3].

Evaluation of the external radiation dose is essential for the judgment of evacuation directive in a nuclear accident. We have newly developed an external environmental dose evaluation system to estimate the radiation dose from a source term in a nuclear facility as a function of the SPEEDI network system. With this system, which is composed of local clients and central server, a realistic external dose proˆle can be quickly evaluated by correcting the dose calculated already with measured values during the monitoring. By the end of the 2006ˆscal year, we developed and validated the system that could be applied to almost all of the LWRs and principal nuclear fuel cycle facilities in Japan.