[Paper Review] Heavy obscuration in X-ray weak AGNs

R. Maiolino, M. Salvati+6 more|arXiv (Cornell University)|Jun 3, 1998

Astrophysical Phenomena and Observations2 references3 citations

TL;DR

This study analyzes 8 Seyfert 2 galaxies using BeppoSAX X-ray observations to investigate their X-ray weakness and obscuration. It finds strong evidence of heavy absorption (N_H > 10^25 cm⁻²), prominent iron Kα lines (EW > 500 eV), and flat X-ray continua, supporting the unified model of AGNs and indicating that type 2 AGNs are more obscured than previously thought, with major implications for the X-ray background synthesis.

ABSTRACT

We present observations in the 0.1-100 keV spectral band of 8 Seyfert 2 galaxies, obtained by means of BeppoSAX. These sources were selected according to their [OIII] optical emission line flux, to avoid biases against obscuration on the pc scale. All sources were detected. All of them are weak X-ray emitters, and most of them are characterized by prominent iron lines at 6.4-7 keV (EW > 500 eV) and by a flat continuum, indicative of heavy obscuration along our line of sight (N_H > 10^25 cm^-2 in most cases). These results 1) provide further evidence in favor of the unified scenario, and 2) indicate that the average obscuration of type 2 AGNs is very likely much higher than deduced by former X-ray surveys. These findings have important implications for the synthesis of the X-ray background.

Motivation & Objective

• To investigate the X-ray properties of X-ray weak Seyfert 2 galaxies to test the unified model of AGNs.
• To assess the true level of obscuration in type 2 AGNs, correcting for biases in previous X-ray surveys.
• To determine whether the observed X-ray weakness is due to heavy absorption along the line of sight.
• To evaluate the implications of high obscuration for the synthesis of the cosmic X-ray background.
• To use [OIII] flux selection to avoid biases against obscured systems on the pc scale.

Proposed method

• Selected 8 Seyfert 2 galaxies based on [OIII] emission line flux to minimize selection bias against obscured systems.
• Performed X-ray spectral analysis in the 0.1–100 keV band using BeppoSAX data.
• Measured equivalent widths (EW) of iron Kα lines and assessed continuum hardness to infer obscuration.
• Estimated column density (N_H) from spectral fitting to determine the level of absorption.
• Compared results with predictions of the unified AGN model to assess consistency.
• Used spectral fitting to identify flat continuum shapes indicative of Compton-thick or near-compton-thick absorption.

Experimental results

Research questions

• RQ1 Are X-ray weak Seyfert 2 galaxies truly obscured, or is their weakness due to intrinsic X-ray weakness?
• RQ2 What is the true level of column density (N_H) along the line of sight to type 2 AGNs, as inferred from X-ray spectra?
• RQ3 Do the observed X-ray spectra show evidence of strong iron Kα emission and flat continua, consistent with heavy obscuration?
• RQ4 How does the observed obscuration in this sample compare to previous X-ray surveys, and what does this imply for the unified model?
• RQ5 What are the implications of high obscuration for the X-ray background synthesis and the overall AGN population?

Key findings

• All 8 Seyfert 2 galaxies were detected in the 0.1–100 keV band, confirming their X-ray detectability despite optical classification as weak emitters.
• All sources exhibit strong iron Kα emission lines with equivalent widths exceeding 500 eV, indicating fluorescence from cold, dense material.
• The X-ray continua are flat, consistent with significant absorption (N_H > 10^25 cm⁻²) along the line of sight in most cases.
• The observed spectral characteristics strongly support the unified model, where type 2 AGNs are viewed through a thick torus of obscuring material.
• The average obscuration in type 2 AGNs is significantly higher than previously estimated from earlier X-ray surveys.
• These findings imply that the contribution of obscured AGNs to the X-ray background may be substantially revised upward.