Probing the Structure and Evolution of BASS Active Galactic Nuclei through Eddington Ratios

We constrain the intrinsic Eddington ratio (λ_Edd) distribution function for local active galactic nuclei (AGN) in bins of low and high obscuration (log NH ≤ 22 and 22 < log NH < 25), using the Swift Burst Alert Telescope 70 month/BASS DR2 survey. We interpret the fraction of obscured AGN in terms of circumnuclear geometry and temporal evolution. Specifically, at low Eddington ratios (log λ_Edd < -2), obscured AGN outnumber unobscured ones by a factor of ~4, reflecting the covering factor of the circumnuclear material (0.8, or a torus opening angle of ~34°). At high Eddington ratios (log λ_Edd > -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. These findings indicate that a revision of the pure geometric unified theory of AGN is necessary to explain the discrepancies between the intrinsic obscured and unobscured AGN populations - especially at high Eddington ratios. We suggest that instead of pure geometric unification, the high λ_Edd regime is in a temporal evolution phase, with obscured AGN transitioning into unobscured phase. We also find that nearly all obscured AGN at log λ_Edd > -1 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 at high λ_Edd, making the latter visible from all lines of sight.