1 research outputs found
Probing the Structure and Evolution of BASS AGN through Eddington Ratios
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.Comment: Accepted by ApJ