Chie Kira

Suzaku-WAM soft gamma-ray all-sky monitor by the earth occultation technique

Suzaku Wide-band All-sky Monitor (WAM), which is the BGO active shield surrounding the Hard X-ray Detector (HXD) onboard Suzaku, has detected many gamma-ray bursts, soft gamma-ray repeaters, and solar flares, since the launch in 2005. In addition, thanks to its large field of view (2 πstr), WAM is able to monitor the bright soft gamma-ray sources by the earth occultation technique in 100–800 keV, as CGRO/BATSE and Fermi/GBM. This is a great advantage to search nonthermal components and observe the hard tail of black hole binaries. In this contribution, we present results on Crab Nebula and Cyg X-1 over 8 years. The long-term light curves of these objects are successfully obtained. Together with light curves of Swift/BAT and Fermi/GBM, we derive the history of broad-band X-ray spectra of these objects in 15–600 keV. The spectral transition of Cyg X-1 is clearly observed but a hard excess above 400 keV reported by INTEGRAL was not significant.

Current Status of the Suzaku Wide‐band All‐sky Monitor (WAM)

The Suzaku Wide‐band All‐sky Monitor (WAM) consists of 20 BGO anti‐coincidence scintillators for the Hard X‐ray Detector (HXD). The WAM has a wide field of view (FOV), about half of the whole sky, a large collecting area, 800 cm2, and broad‐band energy coverage from 50 to 5000 keV. Thus it has been designed to work as a gamma‐ray burst detector. For the three years since Suzaku launch in July 2005, the WAM has been working very well. About 500 GRBs have been detected through the end of 2008, corresponding to a detection rate of ∼140 GRBs per year. The current status of the WAM is presented in this paper.

New probes of GRB prompt emission properties using wide-band spectroscopy by Suzaku Wide-band All-sky Monitor

Although the afterglow observations in HETE‐2 and Swift era have revealed a lot of afterglow properties of gamma‐ray bursts (GRBs), we still have poor understanding of the prompt gamma‐ray emission, such as the emission mechanism of the prompt emission and differences between short and long duration GRBs. We have observed many prompt emission of GRBs by Suzaku Wide‐band All‐sky Monitor in wide energy range of 50–5000 keV, with very large effective area of 400 cm2 even at 1 MeV. Furthermore, a combination of the Suzaku/WAM and Swift data provides us not only wider energy range of 15–5000 keV but also redshift information even for some short GRBs. Thanks to these information, we can firstly investigate an intrinsic correlation for short GRBs like Epeak−Liso relation, and we can derive the same type of relation for time‐resolved spectra of long GRBs in finer time‐scale with higher statistics than ever before. These results could be used to discuss the differences between short and long GRBs, and our time‐res...

Status of GRB Observations with the Suzaku Wideband All‐sky Monitor

The Wide‐band All‐sky Monitor (WAM) is a made up of the large lateral BGO shield of the Hard X‐ray Detector (HXD) onboard Suzaku. Its large geometrical area of 800 cm2 per side, the large stopping power for the hard X‐rays and the wide‐field of view make the WAM an ideal detector for gamma‐ray bursts (GRBs) observations in the energy range of 50–5000 keV. In fact, the WAM has observed 288 GRBs confirmed by other satellites, till the end of May 2007.

The spectral properties of the GRB prompt gamma‐ray emission observed by the Suzaku Wide‐band All‐sky Monitor

We report on the observational results of GRBs by the Suzaku Wide‐band All‐sky Monitor (WAM) in these two years since the Suzaku launch. Using the WAM data, we can investigate the spectral properties of the prompt emission of GRBs with a wider energy band and the highest sensitivity than any previous GRB missions. We found that the spectral properties between short and long GRBs, such as the spectral parameter distribution, the hardness ratio, the spectral lag, and the total emitting energy are clearly different even in the MeV energy band. This result implies that different progenitors or different bulk Lorentz factors of the ejecta are likely causes for the difference between these two classes.We also found that there is a strong correlation between the peak energy and the isotropic equivalent luminosity of the time‐resolved spectra of the bright long GRB 061007, and found that this correlation can be separated well between rising and decay phase of each pulse. This indicates that the physical condition...