Detection of the Intermediate-width Emission Line Region in Quasar OI
  287 with the Broad Emission Line Region Obscured by the Dusty Torus
  The existence of intermediate-width emission line regions (IELRs) in active
galactic nuclei has been discussed for over two decades. A consensus, however,
is yet to be arrived at due to the lack of convincing evidence for their
detection. We present a detailed analysis of the broadband spectrophotometry of
the partially obscured quasar OI 287. The ultraviolet intermediate-width
emission lines (IELs) are very prominent, in high contrast to the corresponding
broad emission lines (BELs) which are heavily suppressed by dust reddening.
Assuming that the IELR is virialized, we estimated its distance to the central
black hole of $\sim 2.9$ pc, similar to the dust sublimation radius of $\sim
1.3$ pc. Photo-ionization calculations suggest that the IELR has a hydrogen
density of $\sim 10^{8.8}-10^{9.4} ~ \rm cm^{-3}$, within the range of values
quoted for the dusty torus near the sublimation radius. Both its inferred
location and physical conditions suggest that the IELR originates from the
inner surface of the dusty torus. In the spectrum of this quasar, we identified
only one narrow absorption-line system associated with the dusty material. With
the aid of photo-ionization model calculations, we found that the obscuring
material might originate from an outer region of the dusty torus. We speculate
that the dusty torus, which is exposed to the central ionizing source, may
produce IELs through photo-ionization processes, while also obscure BELs as a
natural "coronagraph". Such a "coronagraph" could be found in a large number of
partially obscured quasars and be a useful tool to study IELRs.
