The COSMOS field is a well-studied extragalactic field with rich multi-wavelength data (particularly in X-rays and IR) and is one of the two HSC-SSP UltraDeep (UD) fields. The COSMOS UD field was photometrically monitored for 0.5 years in 2016 as a part of the HSC-SSP transient survey. Here we propose to re-observe the COSMOS UD field with HSC g, r, i, and z bands to construct an unprecedentedly deep and wide optical variability-selected AGN sample. The proposed additional single-epoch g, r, i, and z band data in 2024 will extend the time baseline of the COSMOS UD observations to ∼ 8 years. This extension will significantly improve the completeness of the variability-based AGN selection down to i=24 mag and enable us to build a redshift/luminosity less-biased sample of type 1 AGNs in the COSMOS UD field. Using the HSC variability-selected AGN sample combined with multi-wavelength data, we will (1) examine type 1 AGN fraction among X-ray selected AGNs by comparing the variability-selected AGN catalog with the X-ray source catalog, and (2) find a statistical sample of very low black hole mass AGNs and examine the AGN occupation fraction in dwarf galaxies to put an observational constraint on supermassive black hole seed formation models.
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