The swift bat X-ray survey.: III.: X-ray spectra and statistical properties

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 a 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 +/- 1.9 x 10(-3) AGNs deg(-2) (or 396 +/- 80 AGNs all-sky) above a flux level of 2 x 10(-11) ergs cm(-2) s(-1) (similar to 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 subsample of 24 extragalactic sources above the 4.5 sigma detection limit is used to study the statistical properties of AGNs. This sample is composed of local Seyfert galaxies (z = 0: 026, median value) and similar to 10% blazars. We find that 55% of the Seyfert galaxies are absorbed by column densities of N-H > 10(22) H atoms cm(-2) but that none is genuinely bona fide Compton thick. This study shows the capabilities of BAT to probe the hard X-ray sky to the millicrab level.