BAT AGN Spectroscopic Survey--VIII. Type 1 AGN With Massive Absorbing Columns

TL;DR

This study investigates the link between optical obscuration and X-ray absorption in a large sample of AGN, revealing most Type 1 AGN are unabsorbed in X-rays, but a notable fraction shows significant X-ray absorption due to the broad line region.

Contribution

It provides a systematic comparison of optical and X-ray obscuration in AGN, highlighting the small-scale origin of X-ray absorbing gas and the presence of X-ray absorption in some Type 1 AGN.

Findings

86% of Type 1 AGN are X-ray unabsorbed

14% of Type 1 AGN are X-ray absorbed, often due to the broad line region

X-ray absorption fraction remains stable across luminosities and Eddington ratios

Abstract

We explore the relationship between X-ray absorption and optical obscuration within the BAT AGN Spectroscopic Survey (BASS) which has been collecting and analyzing the optical and X-ray spectra for 641 hard X-ray selected ($E>14$ keV) active galactic nuclei (AGN). We use the deviation from a linear broad H$\alpha$-to-X-ray relationship as an estimate of the maximum optical obscuration towards the broad line region and compare the $A_{\rm V}$ to the hydrogen column densities ($N_{\rm H}$) found through systematic modeling of their X-ray spectra. We find that the inferred columns implied by $A_{\rm V}$ towards the broad line region (BLR) are often orders of magnitude less than the columns measured towards the X-ray emitting region indicating a small scale origin for the X-ray absorbing gas. After removing 30\% of Sy 1.9s that potentially have been misclassified due to outflows, we find that 86\% (164/190) of the Type 1 population (Sy 1--1.9) are X-ray unabsorbed as expected based on a single obscuring structure. However, 14\% (26/190), of which 70\% (18/26) are classified as Sy 1.9, are X-ray absorbed, suggesting the broad line region itself is providing extra obscuration towards the X-ray corona. The fraction of X-ray absorbed Type 1 AGN remains relatively constant with AGN luminosity and Eddington ratio, indicating a stable broad line region covering fraction.