Long-term optical variability properties of blazars in the SDSS Stripe 82

The temporal and spectral variabilities of 31 blazars are investigated using the multi-epoch photometric data released in the Sloan Digital Sky Survey (SDSS) Stripe 82. The performed chi-square tests indicate that all the objects show significant brightness variabilities in the Stripe 82 observational period. Two analysis methods, Spearman correlation analysis and linear regression taking into account measurement errors in both the dependent and independent variable, are combined to probe the spectral variability characteristics. As a result, 21 out of 31 blazars exhibit a significant bluer-when-brighter (BWB) trend, which is at least partly attributed to the effect of larger variability amplitude at higher frequencies. Our analysis shows that the BWB trend generally exists for the optical emission of BL Lacs on long time scales. Meanwhile, there are two sources (SDSS J211817.39+001316.8, SDSS J231000.83-000516.2) showing the opposite behavior, i.e., redder-when-brighter (RWB) trend, which might be attributed to the contribution of thermal emission from the accretion disk. Moreover, it is tentatively found that BL Lacs tend to show larger-amplitude brightness variability but smaller-amplitude spectral index variability with respect to FSRQs, implying that there are different physical conditions in these two populations.