在这三篇关于Swift/BAT硬X射线测量（BXS）的系列论文的最后一部分，我们重点讨论了源样品的X射线光谱分析和统计特性。使用一个专门的方法来提取BAT源的时间平均光谱，我们表明，银河系的来源，一般来说，软比河外天体的光谱和赛弗特2星系比赛弗特1 s更难。所有塞弗特星系的平均光谱符合幂律，光子指数为2.00（+/-0.07）。在14-170 keV波段的河外源的累积通量-数关系最好用幂律来描述，斜率α =1.55（+/-0.20），在通量水平为2 $\times 10^{-11}$ erg cm $^{-2}$ s $^{-1}$（~0.85 mCrab）以上，归一化为9.6 $\pm1.9 \times 10^{-3}$ AGN deg $^{-2}$（或396（+/-80）AGN全天）。单位面积累积通量的积分表明BAT可分辨14-170 keV波段中1-2%的X射线背景发射。一个子样本的24个河外源以上的4.5西格玛检测限被用来研究活动星系核的统计特性。这个样本包括本地塞弗特星系（z=0.026，中值）和~10%的耀变体。我们发现55%的Seyfert星系被Log（N_H）>22的柱密度所吸收，但没有一个是真正的康普顿厚度。这项研究表明BAT探测硬X射线天空的能力达到mCrab水平。
英文摘要：
In this concluding part of the series of three papers dedicated to the Swift/BAT hard X-ray survey (BXS), we focus on the X-ray spectral analysis and statistical properties of the source sample. Using a dedicated method to extract time-averaged spectra of BAT sources we show that Galactic sources have, generally, softer spectra than extragalactic objects and that Seyfert 2 galaxies are harder than Seyfert 1s. The averaged spectrum of all Seyfert galaxies is consistent with a power-law with photon index of 2.00 (+/-0.07). The cumulative flux-number relation for the extragalactic sources in the 14-170 keV band is best described by a power-law with a slope alpha=1.55 (+/-0.20) and a normalization of 9.6$\pm1.9 \times 10^{-3}$ AGN deg$^{-2}$ (or 396(+/-80) AGN all-sky) above a flux level of 2$\times 10^{-11}$erg cm$^{-2}$ s$^{-1}$ (~0.85 mCrab). The integration of the cumulative flux per unit area indicates that BAT resolves 1-2% of the X-ray background emission in the 14-170 keV band. A sub-sample of 24 extragalactic sources above the 4.5 sigma detection limit is used to study the statistical properties of AGN. This sample comprises local Seyfert galaxies (z=0.026, median value) and ~10% blazars. We find that 55% of the Seyfert galaxies are absorbed by column densities of Log(N_H)>22, but that none is a bona fide Compton-thick. This study shows the capabilities of BAT to probe the hard X-ray sky to the mCrab level.
