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 MB, 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 ∆m ≈ 0.22 mag, while ∆m ≈ 0.18 mag for the radio-quiet AGNs and ∆m ≈ 0.26 mag for radio-loud AGNs. The ensemble variability for the radio-quiet AGNs shows no significant MB-dependence, while showing positive MB-dependence for the radio-loud AGNs. In any samples the measured variability shows positive correlation among different passbands, with the correlation coefficients of rJH , rHK ′, and rJK ′ ranging from 0.6 to 0.9. For radio-quiet AGNs, the coefficient rHK ′ in a redshift range of 0.1 < z < 0.3 is significantly higher than rJH or rJK ′. 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 non-thermal variable component is suggested as a viable candidate for causing their large and fast variability in the near-infrared region. Subject headings: galaxies: active—quasars: general—galaxies: photometry