The optical spectral variations of 37 blazars with SMARTS data

The variability of blazars remains an open problem. In this work, we use the simultaneous multi-frequency photometric data of Small and Moderate Aperture Research Telescope System (SMARTS) to study the optical spectral variability of 37 blazars. Our sample includes 29 flat-spectrum radio quasars (FSRQs), 6 BL Lacertae objects (BL Lacs), and 2 blazar candidates of unknown type. By performing the linear fits in the plane of the spectral index versus optical flux, we obtain the slope K for each source, which characterizes its optical spectral behavior. Based on K, 13 sources exhibit a bluer-when-brighter (BWB) trend (K > 0.1), 14 sources show a redder-when-brighter (RWB) trend (K < −0.1), and 10 sources display achromatic behavior (−0.1 < K < 0.1). Most FSRQs favor the RWB trend, while most BL Lacs tend to show the BWB trend. We find a weak positive correlation between K and log νp, and a weak negative correlation between K and log νp. Based on the criteria ΔBIC > 10, we select 7 targets with significant trend transition behavior, and reproduce the non-linear variation behavior quantitatively via the two-component model with the modulation of the intrinsic peak frequency. Our work elucidates the variation mechanism of blazars at optical bands.