Probing the Structure and Evolution of BASS AGN through Eddington Ratios

TL;DR

This study investigates how the distribution of Eddington ratios in local AGN relates to their obscuration, revealing that high Eddington ratios correspond to less obscuration due to rapid clearing of the circum-nuclear material.

Contribution

It provides new insights into the relationship between Eddington ratios and obscuration, combining observational data with models of torus clearing to interpret AGN evolution.

Findings

Obscured AGN outnumber unobscured at low Eddington ratios by a factor of ~4.

At high Eddington ratios, less than 30% of AGN are obscured, indicating rapid clearing of the torus.

Nearly all high-luminosity obscured AGN show broad emission lines, suggesting a geometric change.

Abstract

We constrain the intrinsic Eddington ratio (\lamEdd ) distribution function for local AGN in bins of low and high obscuration (log NH <= 22 and 22 < log NH < 25), using the Swift-BAT 70-month/BASS DR2 survey. We interpret the fraction of obscured AGN in terms of circum-nuclear geometry and temporal evolution. Specifically, at low Eddington ratios (log lamEdd < -2), obscured AGN outnumber unobscured ones by a factor of ~4, reflecting the covering factor of the circum-nuclear material (0.8, or a torus opening angle of ~ 34 degrees). At high Eddington ratios (\log lamEdd > -1), the trend is reversed, with < 30% of AGN having log NH > 22, which we suggest is mainly due to the small fraction of time spent in a highly obscured state. Considering the Eddington ratio distribution function of narrow-line and broad-line AGN from our prior work, we see a qualitatively similar picture. To disentangle temporal and geometric effects at high lamEdd, we explore plausible clearing scenarios such that the time-weighted covering factors agree with the observed population ratio. We find that the low fraction of obscured AGN at high lamEdd is primarily due to the fact that the covering factor drops very rapidly, with more than half the time is spent with < 10% covering factor. We also find that nearly all obscured AGN at high-lamEdd exhibit some broad-lines. We suggest that this is because the height of the depleted torus falls below the height of the broad-line region, making the latter visible from all lines of sight.