The longer optical time scales of a large sample of quasars

Of a carefully studied sample of 60 quasars, 75% displayed evidence of long-term changes in the base levels of their optical fluxes. Internally consistent light curves were analyzed by three different methods in an effort to assign time scales to these changes. The familiar rapid, large-amplitude flares were specifically excluded from the study. The majority of the observed time scales range from 3 to 15 yr, but when they are translated into the rest frames of the objects, there is a strong concentration between 2 and 6 yr. Extensive testing showed that the lengths of the data runs, largely 17 to 21 yr, did not exert a major influence on the estimation of the time scales. Several accepted source mechanisms appear to be capable of generating variations with appropriate time scales.