Suzaku View of the Swift/BAT Active Galactic Nuclei. I. Spectral Analysis of Six Active Galactic Nuclei and Evidence for Two Types of Obscured Population

Abstract

We present a systematic spectral analysis with Suzaku of six active galactic nuclei (AGNs) detected in the Swift/BAT hard X-ray (15-200 keV) survey: Swift J0138.6-4001, J0255.2-0011, J0350.1-5019, J0505.7-2348, J0601.9-8636, and J1628.1-5145. This is considered to be a representative sample of new AGNs without X-ray spectral information before the BAT survey. We find that the 0.5-200 keV spectra of these sources can be uniformly fitted with a base model consisting of heavily absorbed (log N _H > 23.5 cm^-2) transmitted components, scattered lights, a reflection component, and an iron-K emission line. There are two distinct groups: three "new-type" AGNs (including the two sources reported by Ueda et al.) with an extremely small scattered fraction (f _scat < 0.5%) and strong reflection component (R = Ω/2π gsim 0.8, where Ω is the solid angle of the reflector), and three "classical-type" ones with f _scat > 0.5% and R lsim 0.8. The spectral parameters suggest that the new type has an optically thick torus for Thomson scattering (N _H ~ 10²⁵ cm^-2) with a small opening angle θ ~ 20° viewed in a rather face-on geometry, while the classical type has a thin torus (N _H ~ 10^23-24 cm^-2) with θ gsim 30°. We infer that a significant number of new-type AGNs with an edge-on view are missing in the current all-sky hard X-ray surveys.