2020ApJ...889..110K

We have performed a systematic study of gamma-ray bursts (GRBs), which have various values in the peak energy of the νF_ν spectrum of prompt emission, E_peak, observed by the Swift/Burst Alert Telescope (BAT) and Fermi/Gamma-ray Burst Monitor, investigating their prompt and X-ray afterglow emissions. We cataloged long-lasting GRBs observed by Swift between 2004 December and 2014 February in three categories according to the classification by Sakamoto et al.: X-ray flashes (XRFs), X-ray-rich GRBs (XRRs), and classical GRBs (C-GRBs). We then derived E_peak^obs, as well as E_peak^src if viable, of the Swift spectra of their prompt emission. We also analyzed their X-ray afterglows and found that GRB events with a lower E_peak^src, i.e., softer GRBs, are fainter in 0.3-10 keV X-ray luminosity and decay more slowly than harder GRBs. The intrinsic event rates of the XRFs, XRRs, and C-GRBs were calculated using the Swift/BAT trigger algorithm. Those of the XRRs and XRFs are larger than that of the C-GRBs. If we assume that the observational diversity of E_peak is explained using the off-axis model, these results yield a jet half-opening angle of Δθ ∼ 0.°3, and a variance of the observing angles θ_obs <= 0.°6. This implies that this tiny variance would be responsible for the E_peak diversity observed by Swift/BAT, which is unrealistic. Therefore, we conclude that the E_peak diversity is not explained with the off-axis model, but is likely to originate from some intrinsic properties of the jets.