On Spectral Peak Energy of Swift Gamma-Ray Bursts

Abstract

Owing to narrow energy band of \textit{Swift}/BAT, several urgent issues are required to pay more attentions but unsolved so far. We systematically study the properties of a refined sample of 283 \textit{Swift}/BAT gamma-ray bursts with well-measured spectral peak energy ($E_{\text{p}}$) at a high confidence level larger than 3$\sigma$. It is interestingly found that duration ($T_{90}$) distribution of \textit{Swift} bursts still exhibits an evident bimodality with a more reliable boundary of $T_{90}\simeq$1.06 s instead of 2 s for previously contaminated samples including bursts without well-peaked spectra, which is very close to $\sim$1.27 s and $\sim$0.8 s suggested by some authors for Fermi/GBM and \textit{Swift}/BAT catalogs, respectively. The \textit{Swift}/BAT short and long bursts have comparable mean $E_{\text{p}}$ values of $87_{-49}^{+112}$ and $85_{-46}^{+101}$ keV in each, similar to what found for both types of BATSE bursts, which manifests the traditional short-hard/long-soft scheme may not be tenable for the certain energy window of a detector. In statistics, we also investigate the consistency of distinct methods for the $E_{\text{p}}$ estimates and find that Bayesian approach and BAND function can always give consistent evaluations. In contrast, the frequently-used cut-off power-law model matches two other methods for lower $E_{\text{p}}$ and will overestimate the $E_{\text{p}}$ more than 70\% as $E_{\text{p}}>$100 keV. Peak energies of X-ray flashes, X-ray rich bursts and classical gamma-ray bursts could have an evolutionary consequence from thermal-dominated to non-thermal-dominated radiation mechanisms. Finally, we find that the $E_{\text{p}}$ and the observed fluence ($S_{\gamma }$) in the observer frame are correlated as $E_p\simeq [S_{\gamma }/(10^{-5}ergc{m}^{-2}){]}^{0.28}\times 117.5_{-32.4}^{+44.7}$ keV proposed to be an useful indicator of GRB peak energies.

arXiv comment25 pages; 7 figures; 2 tables. Accepted for publication in ApJ

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