On the Spectral Peak Energy of Swift Gamma-Ray Bursts

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 sigma. We find that the duration (T-90) distribution of Swift bursts still exhibits an evident bimodality with a more reliable boundary of T-90 similar or equal to 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 similar to 1.27 and similar to 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 87 (+112)(-49) and 85(46)(+101) 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 forE(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 lowerE(p)and overestimates theE(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-gamma) in the observer frame are correlated as E-p similar or equal to inverted right perpendicularS gamma/(10(-5) erg cm(-2))inverted left perpendicular(0.28) x 117.5(-32.4+)(44.7) keV, which might be a useful indicator of GRB peak energies.