On the Spectral Peak Energy of Swift Gamma-Ray Bursts
Z. B. Zhang, M. Jiang, Y. Zhang, K. Zhang, X. J. Li, Q. Zhang
Abstract
Abstract Owing to the narrow energy band of the Swift Burst Alert Telescope (BAT), several urgent issues remain unsolved. We systematically study the properties of a refined sample of 283 Swift/BAT gamma-ray bursts (GRBs) with well-measured spectral peak energy ( E p ) at a high confidence level greater than 3 σ . We find that the duration ( T 90 ) distribution of Swift bursts still exhibits an evident bimodality with a more reliable boundary of T 90 ≃ 1.06 s instead of 2 s as found for previously contaminated samples, including bursts without well-peaked spectra, which is very close to the ∼1.27 and ∼0.8 s values suggested in the literature for the Fermi/Gamma-ray Burst Monitor and Swift/BAT catalogs, respectively. The Swift/BAT short and long bursts have comparable mean E p values of and keV, similar to what was found for both types of BATSE bursts, which indicates that the traditional short–hard/long–soft scheme may not be tenable for certain detector energy windows. We also statistically investigate the consistency of distinct methods for E p estimates and find that a Bayesian approach and BAND function (Band et al.) can always provide consistent evaluations. In contrast, the frequently used cutoff power-law model matches two other methods for lower E p and overestimates the E p by more than 70%, as E p > 100 keV. Peak energies of X-ray flashes, X-ray-rich bursts, and classical GRBs could be an evolutionary consequence of moving from thermal-dominated to nonthermal-dominated radiation mechanisms. Finally, we find that the E p and the observed fluence ( S γ ) in the observer frame are correlated as keV, which might be a useful indicator of GRB peak energies.