Spectral properties of gamma-ray bursts observed by the Suzaku wide-band all-sky monitor
Yamaoka, Kazutaka; Fukazawa, Yasushi; Kokubun, Motohide; Makishima, Kazuo; Nakagawa, Yujin E.; Nakazawa, Kazuhiro; Ohmori, Norisuke; Ohno, Masanori; Sugita, Satoshi; Takahashi, Tadayuki; Tashiro, Makoto S.; Terada, Yukikatsu; Urata, Yuji; Yamauchi, Makoto; Hurley, Kevin; Iwakiri, Wataru; Katsukura, Daisuke; Murakami, Souta; Odaka, Katsuya; Takahashi, Kaito
Japan, Taiwan, United States
Abstract
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 100-5000 keV range integrated over the entire emission and the 1 s 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 α 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 (α and β) for the entire emission spectra were consistent with previous measurements. The averages of the α and β were -0.90 and -2.65 for long-duration GRBs, while the average α was -0.55 and the β was not well constrained for short-duration GRBs. However, the average peak energy Epeak was 645 and 1286 keV for long- and short-duration GRBs respectively, which are higher than previous Fermi/GBM measurements (285 keV and 736 keV). The α and Epeak of the 1 s 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 Epeaks measured by the Suzaku WAM could be due to detector selection bias, mainly caused by the limited energy range above 100 keV.