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X-ray emission detection in a galaxy is one of the efficient tools forselecting Active Galactic Nuclei (AGNs). However, many X-ray-selected AGNs arenot easily selected as AGNs by their optical emission. These galaxies,so-called optically dull (OD) AGNs, are fascinating since their X-ray emissionis bright even though the AGN signature in the optical regime is absent. In adeep 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 withoptical spectra as AGN using X-ray selection up to redshift $z\sim 1.5$. Weinspected the spectra to check for any AGN signature in their optical emissionlines: [Ne V] forbidden emission line, Mass Excitation diagram (MEx), colorexcitation diagram (TBT), and excess in [O II ] emission line. Finally, wefound 48 AGNs show AGN signatures in the optical spectrum classified asnarrow-line AGN and 180 AGNs that did not show any AGN signature as OD AGNsample. The simple explanation of OD AGN's nature is due to a bright hostgalaxy 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. Atthe 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 AGNshave a lower accretion rate of $(\lambda_\text{Edd})\lesssim 10^{-2}$ than thetypical AGN value. We expected the lower accretion rate sources that sufferfrom neither host galaxy dilution nor obscuration to have RadiativelyInefficient Flow (RIAF) in their accretion disk. Finally, nine sources havebeen identified to be most likely host the RIAF disk.

X-ray Selected Narrow-Line Active Galactic Nuclei in the COSMOS Field: Nature of Optically Dull Active Galactic Nuclei