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

We constrain the intrinsic Eddington ratio (λEdd) 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 λEdd, 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 λEdd 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-λEdd 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.