$JHK'$ Imaging Photometry of Seyfert 1 AGNs and Quasars III: Variability of Radio Quiet and Radio Loud AGNs

Variability of 226 AGNs in the near-infrared $J$, $H$, and $K'$ bands is analyzed and discussed. An ensemble average for measured variabilities was obtained for various samples of the AGNs divided by absolute $B$-magnitude $M_B$, redshift $z$, and radio strength. All the samples in the $J$, $H$, and $K'$ bands are found to give significant ensemble variability, but no significant wavelength dependence is found. The ensemble variability in the entire sample combining the $J$, $H$, and $K'$ samples is $\Delta m\approx 0.22$ mag, while $\Delta m\approx 0.18$ mag for the radio-quiet AGNs and $\Delta m\approx 0.26$ mag for radio-loud AGNs. The ensemble variability for the radio-quiet AGNs shows no significant $M_B$-dependence, while showing positive $M_B$-dependence for the radio-loud AGNs. In any samples the measured variability shows positive correlation among different passbands, with the correlation coefficients of $r_{JH}$, $r_{HK'}$, and $r_{JK'}$ ranging from 0.6 to 0.9. For radio-quiet AGNs, the coefficient $r_{HK'}$ in a redshift range of $0.1<z<0.3$ is significantly higher than $r_{JH}$ or $r_{JK'}$. The coefficient for the radio-loud AGNs with $0.6<z0.3$ and radio-loud AGNs with $z<0.3$, we cannot confirm such strong correlation among different passbands. All the features of near-infrared variability for the radio-quiet AGNs are consistent with a simple dust reverberation model of the central regions of AGNs. However, the features for the radio-loud AGNs are not fully explained by such a model, and a non-thermal variable component is suggested as a viable candidate for causing their large and fast variability in the near-infrared region.