Suzaku Observations of Active Galactic Nuclei Detected in the Swift/BAT Survey: Discovery of "New Type" of Buried Supermassive Black Holes

TL;DR

This study reports Suzaku observations of two newly identified heavily obscured AGNs from the Swift/BAT survey, revealing a new class of buried supermassive black holes with unique geometrical and scattering properties.

Contribution

It introduces a new class of heavily buried AGNs with minimal scattered light, suggesting a different torus geometry and indicating many such objects remain undetected.

Findings

Both AGNs show high column density and strong reflection components.

They exhibit extremely low scattered light fractions (<0.5%).

Implication of a large population of similar hidden AGNs in the local universe.

Abstract

We present the Suzaku broad band observations of two AGNs detected by the Swift/BAT hard X-ray (>15 keV) survey that did not have previous X-ray data, Swift J0601.9-8636 and Swift J0138.6-4001. The Suzaku spectra reveals in both objects a heavily absorbed power law component with a column density of NH =~ 10^{23.5-24} cm^{-2} that dominates above 10 keV, and an intense reflection component with a solid angle >~ $2\pi$ from a cold, optically thick medium. We find that these AGNs have an extremely small fraction of scattered light from the nucleus, <~ 0.5% with respect to the intrinsic power law component. This indicates that they are buried in a very geometrically-thick torus with a small opening angle and/or have unusually small amount of gas responsible for scattering. In the former case, the geometry of Swift J0601.9-8636 should be nearly face-on as inferred from the small absorption for the reflection component. The discovery of two such objects in this small sample implies that there must be a significant number of yet unrecognized, very Compton thick AGNs viewed at larger inclination angles in the local universe, which are difficult to detect even in the currently most sensitive optical or hard X-ray surveys.