Long-term optical color behavior of a sample of blazars

Abstract

We report on the optical color variability of a sample of 24 blazars, consisting of nine flat spectrum radio quasars (FSRQs) and 15 BL Lacertae objects (BL Lacs), on timescales of years using simultaneous V- and R-band observations observed by the Kanata telescope at Higashi-Hiroshima Observatory. The correlations between color indices V – R and magnitudes reveal that 11 BL Lacs and one FSRQ exhibited significant (i.e., r > 0.2 and P < 0.01) bluer-when-brighter (BWB) trend and two FSRQs followed the redder-when-brighter (RWB) tend, indicating a possibility that the BWB chromatic trend is dominant for BL Lacs and the RWB trend is especially found in FSRQs, which has been presented occasionally in different samples of blazars. The superpositions of the red emission component from the Doppler-boosted relativistic jet and the blue component arising from the accretion disk might be a possible interpretation for the long-term color behaviors.