JHK′ imaging photometry of Seyfert 1 active galactic nuclei and quasars. III. Variability of radio-quiet and radio-loud active galactic nuclei

Variability of 226 active galactic nuclei (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 Δm ≈ 0. 22 mag, while Δm ≈ 0.18 mag for the radio-quiet AGNs and Δm ≈ 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 < z < 1.0 is as high as 0.95, irrespective of the passband. However, for the radio-quiet AGNs with z > 0. 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 nonthermal variable component is suggested as a viable candidate for causing their large and fast variability in the near-infrared region.