X-ray Selected Narrow-Line Active Galactic Nuclei in the COSMOS Field:
  Nature of Optically Dull Active Galactic Nuclei
  X-ray emission detection in a galaxy is one of the efficient tools for
selecting Active Galactic Nuclei (AGNs). However, many X-ray-selected AGNs are
not easily selected as AGNs by their optical emission. These galaxies,
so-called optically dull (OD) AGNs, are fascinating since their X-ray emission
is bright even though the AGN signature in the optical regime is absent. In a
deep multiwavelength survey over 2 deg$^2$ of the Cosmic Evolution Survey
(COSMOS) field, we have looked for the OD AGNs using photometric,
spectroscopic, and X-ray data. We identified 310 non-broad line sources with
optical spectra as AGN using X-ray selection up to redshift $z\sim 1.5$. We
inspected the spectra to check for any AGN signature in their optical emission
lines: [Ne V] forbidden emission line, Mass Excitation diagram (MEx), color
excitation diagram (TBT), and excess in [O II ] emission line. Finally, we
found 48 AGNs show AGN signatures in the optical spectrum classified as
narrow-line AGN and 180 AGNs that did not show any AGN signature as OD AGN
sample. The simple explanation of OD AGN's nature is due to a bright host
galaxy that dilutes the AGN light or dust materials obscuring the AGN light.
The bright host galaxy dilution explains nearly $70\%$ of our OD AGN sample. At
the same time, the dust material obscuration is unlikely for the main reason.
By estimating the Eddington ratio, we also found that 95/180 of our OD AGNs
have a lower accretion rate of $(\lambda_\text{Edd})\lesssim 10^{-2}$ than the
typical AGN value. We expected the lower accretion rate sources that suffer
from neither host galaxy dilution nor obscuration to have Radiatively
Inefficient Flow (RIAF) in their accretion disk. Finally, nine sources have
been identified to be most likely host the RIAF disk.
