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 $[\mathrm{log}({N}_{{\rm{H}}}/{\mathrm{cm}}^{-2})\leqslant 22$ and $22\lt \mathrm{log}({N}_{{\rm{H}}}/{\mathrm{cm}}^{-2})\lt 25$ ], 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 ( $\mathrm{log}\,{\lambda }_{\mathrm{Edd}}\,\lt \,$ -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 ( $\mathrm{log}\,{\lambda }_{\mathrm{Edd}}\,\gt \,$ -1), the trend is reversed, with <30% of AGN having $\mathrm{log}({N}_{{\rm{H}}}/{\mathrm{cm}}^{-2})\gt 22$ , 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 λ _Edd, 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 λ _Edd is primarily due to the fact that the covering factor drops very rapidly, with more than half the time spent with <10% covering factor. We also find that nearly all obscured AGN at high-λ _Edd 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.