Swift/BAT Observations of X-Ray Flashes

Abstract

An analysis of prompt gamma-rays of X-ray flashes (XRFs) observed with the Swift/BAT has been presented. Our sample includes 235 bursts. It is found that the BAT detection ratio of XRFs to typical Gamma-ray bursts (GRBs) is 42:193, for division at Γ = 2 (roughly corresponding Ep~50 keV), Γ being the power law index of the BAT spectrum and Ep, the peak energy (Ep) of the νfν spectrum. This is consistent with the HETE-2 observations. For both XRFs and GRBs Γ are almost normally distributed in the range of 1 to 2.8, similar that observed with HETE-2. The distribution of Γ for the entire set of GRBs/XRFs is not available due to poor statistics on the peak at Γ>2.3. This result probably indicates that the BAT spectrum of a typical XRF could have a Γ of roughly 2.3, if they indeed are a distinct soft component of the GRB population. By comparing the fluence and the peak flux in different energy bands, it is found that the XRFs are ordinarily softer than the GRBs, but during the peak time the spectra of both GRBs and XRF are similar, showing that the dominant radiation mechanisms of both GRBs and XRFs are similar.