The Swift BAT X-Ray Survey. III. X-Ray Spectra and Statistical Properties
Abstract
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 α = 1.55 +/- 0.20 and a normalization of 9.6 +/- 1.9 × 10-3 AGNs deg-2 (or 396 +/- 80 AGNs all-sky) above a flux level of 2 × 10-11 ergs 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 subsample of 24 extragalactic sources above the 4.5 σ 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 ~10% blazars. We find that 55% of the Seyfert galaxies are absorbed by column densities of NH > 1022 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 2008
- DOI:
- 10.1086/524104
- arXiv:
- arXiv:0709.4333
- Bibcode:
- 2008ApJ...673...96A
- Keywords:
-
- galaxies: active;
- surveys;
- X-rays: binaries;
- X-rays: galaxies;
- Astrophysics
- E-Print:
- Accepted for publication in The Astrophysical Journal