Long-term optical-infrared color variability of blazars

TL;DR

This study analyzes long-term optical-infrared color variability in blazars, revealing different variability amplitudes and color trends between FSRQs and BL Lacs, and exploring underlying emission mechanisms.

Contribution

It provides a comprehensive analysis of color variability patterns in a large blazar sample, highlighting differences between FSRQs and BL Lacs and their brightness states.

Findings

FSRQs show larger variability amplitudes than BL Lacs.

Most FSRQs are more variable at lower energy (J band) than at higher energy (R band).

Many blazars exhibit color trends such as redder-when-brighter or bluer-when-brighter.

Abstract

The long-term optical and infrared color variability of blazars has been investigated with the SMARTS monitoring data. The sample in this study consists of 49 flat spectrum radio quasars and 22 BL Lac objects. The fractional variability amplitudes of each source have been calculated in both optical R band and infrared J band. Overall, the variability amplitudes of FSRQs are larger than those of BL Lac objects. The results also suggest that the variability amplitude of most FSRQs is larger at lower energy band (J band) than at higher one (R band), while the variability amplitude of BL Lacs are larger at higher energy band. Two types of blazars both display color variation along with the variability in brightness. However, they show different variation behaviors in general. With the whole data set, 35 FSRQs exhibit redder-when-brighter trends, and 11 FSRQs exhibit opposite trends; 11 BL Lacs follow bluer-when-brighter trends, and 7 BL Lacs follow opposite trends. The examination in detail shows that there are 10 blazars showing redder-when-brighter trend in their low state, and bluer-when-brighter or stable-when-brighter trends in their high state. Some more complicated color behaviors have also been detected in several blazars. The non-thermal jet emission and the thermal emission from the accretion disc are employed to explain the observed color behaviors.