X-ray Spectral Properties of the BAT AGN Sample

TL;DR

This paper analyzes the X-ray spectral properties of local AGN from the Swift BAT catalog, revealing the prevalence of hidden AGN, their host galaxy types, and their contribution to the cosmic X-ray background.

Contribution

It provides the first unbiased X-ray spectral analysis of a complete local AGN sample, including the identification of a significant fraction of hidden AGN and host galaxy characteristics.

Findings

24% of AGN are 'hidden' with low scattering fractions.

Absorbed source fraction increases at lower luminosities.

Average spectrum reproduces the cosmic X-ray background slope.

Abstract

The 9-month Swift Burst Alert Telescope (BAT) catalog provides the first unbiased (log N_H < 24) look at local (<z> = 0.03) AGN. In this paper, we present the collected X-ray properties (0.3 - 12keV) for the 153 AGN detected. In addition, we examine the X-ray properties for a complete sample of non-beamed sources, above the Galactic plane. Of these, 45% are best fit by simple power law models while 55% require the more complex partial covering model. One of our goals was to determine the fraction of "hidden" AGN, which we define as sources with scattering fractions < 0.03 and ratios of soft to hard X-ray flux < 0.04. We found that "hidden" AGN constitute a high percentage of the sample (24%), proving that they are a very significant portion of local AGN. Further, we find that the fraction of absorbed sources does increase at lower unabsorbed 2-10 keV luminosities, as well as accretion rates. Some of the most interesting results for the BAT AGN sample involve the host galaxy properties. We found that 33% are hosted in peculiar/irregular galaxies and only 5/74 hosted in ellipticals. Further, 54% are hosted in interacting/merger galaxies. Finally, we present both the average X-ray spectrum (0.1-10 keV) and log N-log S in the 2-10 keV band. With our average spectrum, we have the remarkable result of reproducing the measured CXB X-ray power law slope of ~ 1.4 (Marshall et al. 1980). From the log N-log S relationship, we show that we are complete to log S < -11 in the 2-10 keV band. Both the collected X-ray properties of our uniform sample and the log N-log S relationship will now provide valuable input to X-ray background models for z ~ 0. (abridged)