The near-infrared variability of a sample of optically selected quasars

A complete sample of 108 optically selected quasars, the Palomar-Green quasars, has been studied at near-infrared wavelengths over an average time base of about 6 yr; in some cases the time base extends to 20 yr. The measurements show that about half the quasars have a high probability of having varied, but that half show little evidence of variability, in sharp contrast to published studies indicating that most quasars vary on comparable timescales at optical wavelengths. The maximum amplitude of variation is less than 1 mag and generally near 0.5 mag. During variations, the near-infrared colors are almost constant. The more luminous quasars show a smaller mean probability of having varied in the time frame of the study than the less-luminous quasars. The majority of the quasars with flat radio spectra have a high probability of having varied. Among those quasars that have a high probability of having varied, the rate is fairly independent of the time interval between the observations after the first year. The quasar 3C 273 shows definite variations at 10.1 µm which preclude the emission being thermal emission from heated dust grains for that quasar. The observations of the other quasars cannot be used to differentiate uniquely between thermal and nonthermal emission mechanisms