R. Hara

Testing the E(sub peak)-E(sub iso) Relation for GRBs Detected by Swift and Suzaku-WAM

One of the most prominent, yet controversial associations derived from the ensemble of prompt-phase observations of gamma-ray bursts (GRBs) is the apparent correlation in the source frame between the peak energy Epeak) of the nu-F(nu) spectrum and the isotropic radiated energy, Eiso. Since most gamma-ray bursts (GRBs) have Epeak above the energy range (15-150 keV) of the Burst Alert Telescope (BAT) on Swift, determining accurate Epeak values for large numbers of Swift bursts has been difficult. However, by combining data from Swift/BAT and the Suzaku Wide-band All-Sky Monitor (WAM), which covers the energy range from 50-5000 keV, for bursts which are simultaneously detected, one can accurately fit Epeak and Eiso and test the relationship between them for the Swift sample. Between the launch of Suzaku in July 2005 and the end of April 2009, there were 48 gamma-ray bursts (GRBs) which triggered both Swift/BAT and WAM and an additional 48 bursts which triggered Swift and were detected by WAM, but did not trigger. A BAT-WAM team has cross-calibrated the two instruments using GRBs, and we are now able to perform joint fits on these bursts to determine their spectral parameters. For those bursts with spectroscopic redshifts, we can also calculate the isotropic energy. Here we present the results of joint Swift/BAT-Suzaku/WAM spectral fits for 91 of the bursts detected by the two instruments. We show that the distribution of spectral fit parameters is consistent with distributions from earlier missions and confirm that Swift bursts are consistent with earlier reported relationships between Epeak and isotropic energy. We show through time-resolved spectroscopy that individual burst pulses are also consistent with this relationship.

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...

Reviewing Epeak Relations with Swift and Suzaku Data

In recent years several authors have derived correlations between gamma‐ray burst (GRB) spectral peak energy (Epeak) and either isotropic‐equivalent radiated energy (Eiso) or peak luminosity (Liso). Since these relationships are controversial, but could provide redshift estimators, it is important to determine whether bursts detected by Swift exhibit the same correlations. Swift has greatly added to the number of GRBs for which redshifts are known and hence Eiso and Liso could be calculated. However, for most bursts it is not possible to adequately constrain Epeak with Swift data alone since many GRBs have Epeak above the energy range (15–150 keV) of the Swift Burst Alert Telescope (BAT). Therefore we have analyzed the spectra of 78 bursts (31 with redshift) which were detected by both Swift/BAT and the Suzaku Wide‐band All‐sky Monitor (WAM), which covers the energy range 50–5000 keV. For most bursts in this sample we can precisely determine Epeak and for bursts with known redshift we can compare how the ...

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...