Properties of AGN Multiband Optical Variability in the HSC SSP Transient
  Survey
  We study variability of active galactic nuclei (AGNs) by using the deep
optical multiband photometry data obtained from the Hyper Suprime-Cam Subaru
Strategic Program (HSC SSP) survey in the COSMOS field. The images analyzed
here were taken with 8, 10, 13, and 15 epochs over three years in the $g$, $r$,
$i$, and $z$ bands, respectively. We identified 491 robust variable AGN
candidates, down to $i=25$ mag and with redshift up to $4.26$. Ninety percent
of the variability-selected AGNs are individually identified with the X-ray
sources detected in the Chandra COSMOS Legacy survey. We investigate their
properties in variability by using structure function analysis and find that
the structure function for low-luminosity AGNs
($L_{\mathrm{bol}}\lesssim10^{45}$ erg s$^{-1}$) shows a positive correlation
with luminosity, which is the opposite trend for the luminous quasars. This
trend is likely to be caused by larger contribution of the host galaxy light
for lower-luminosity AGNs. Using the model templates of galaxy spectra, we
evaluate the amount of host galaxy contribution to the structure function
analysis and find that dominance of the young stellar population is needed to
explain the observed luminosity dependence. This suggests that low-luminosity
AGNs at $0.8\lesssim z\lesssim1.8$ are predominantly hosted in star-forming
galaxies. The X-ray stacking analysis reveals the significant emission from the
individually X-ray undetected AGNs in our variability-selected sample. The
stacked samples show very large hardness ratios in their stacked X-ray
spectrum, which suggests that these optically variable sources have large soft
X-ray absorption by dust-free gas.
