X-ray absorption and 9.7 um silicate feature as a probe of AGN torus structure

Abstract

The dusty torus plays a vital role in unifying active galactic nuclei (AGNs). However, the physical structure of the torus remains largely unclear. Here we present a systematical investigation of the torus mid-infrared (MIR) spectroscopic feature, i.e., the 9.7 um silicate line, of $175$ AGNs selected from the Swift/BAT Spectroscopic Survey (BASS). Our sample is constructed to ensure that each of the $175$ AGNs has Spizter/IRS MIR, optical, and X-ray spectroscopic coverage. Therefore, we can simultaneously measure the silicate strength, optical emission lines, and X-ray properties (e.g., the column density and the intrinsic X-ray luminosity). We show that, consistent with previous works, the silicate strength is weakly correlated with the hydrogen column density ($N_{\mathrm{H}}^{\mathrm{X}}$), albeit with large scatters. For X-ray unobscured AGNs, the silicate-strength-derived $V$-band extinction and the broad-H$\alpha$-inferred one are both small; however, for X-ray obscured AGNs, the former is much larger than the latter. In addition, we find that the optical type 1 AGNs with strong X-ray absorption on average show significant silicate absorption, indicating that their X-ray absorption might not be caused by dust-free gas in the broad-line region. Our results suggest that the distribution and structure of the obscuring dusty torus are likely to be very complex. We test our results against the smooth and clumpy torus models and find evidence in favor of the clumpy torus model.

17 pages, 12 figures, Accepted to Research in Astronomy and Astrophysics (a new version with appropriate top margins so that the few bottom lines appear properly)

References in corpus (12)

• AGN Dusty Tori: I. Handling of Clumpy Media
• BAT AGN Spectroscopic Survey - V. X-ray properties of the Swift/BAT 70-month AGN catalog
• BAT AGN Spectroscopic Survey I: Spectral Measurements, Derived Quantities, and AGN Demographics
• ALMA images the many faces of the NGC1068 torus and its surroundings
• 9.7 um Silicate Features in AGNs: New Insights into Unification Models
• An image of the dust sublimation region in the nucleus of NGC 1068
• Long-Term X-ray Variability of Typical Active Galactic Nuclei in the Distant Universe
• Resolving the AGN and host emission in the mid-infrared using a model-independent spectral decomposition
• Constraints on the Broad Line Region Properties and Extinction in Local Seyferts
• Piercing Through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields: I. X-ray Spectral and Long-term Variability Analyses
• The relationship between X-ray and optical absorbers in active galactic nuclei
• Detection of faint broad emission lines in type 2 AGN: III. On the $M_{BH}-{\sigma }_{\star }$ relation of type 2 AGN

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• AGN Selection and Demographics in GOODS-S/HUDF from X-ray to Radio
• Piercing through Highly Obscured and Compton-thick AGNs in the Chandra Deep Fields. II. Are Highly Obscured AGNs the Missing Link in the Merger-Triggered AGN-Galaxy Coevolution Models?
• Measurement of AGN dust extinction based on the near-infrared flux variability of WISE data
• Updated Picture of the Active Galactic Nuclei with Dusty/Dust-free Gas Structures and Effects of the Radiation Pressure
• Multiwavelength Analysis of a Nearby Heavily Obscured AGN in NGC 449