2023MNRAS.519.5263Z

Spectral variation accompanied by flux variability is a commonly observed phenomenon for blazars. In order to investigate the optical spectral feature of blazars further, we collected the long-term optical V- and R-band data of 27 blazars (14 BL Lacaerte objects, BL Lacs, and 13 flat-spectrum radio quasars, FSRQs) and calculated their optical spectral indices. The results show that the spectral indices vary with respect to brightness for all of these blazars. In general, the optical spectrum progressively becomes flatter (or steeper), when the brightness increases. However the spectrum changes more and more slowly, until it tends to stability. In other words, the source becomes bluer (or redder) and then gradually stabilizes when it brightens, behaviours that are referred to as bluer-stable-when-brighter (BSWB) and redder-stable-when-brighter (RSWB), respectively. A total of 13 of the 14 BL Lacs show the BSWB behaviour, the exception being AO 0235+164. In contrast, most of the FSRQs (10 out of 13) exhibit the RSWB trend. It is confirmed that blazars follow the two universal optical spectral behaviours, namely BSWB and RSWB. The model of two constant-spectral-index components can explain the optical spectral features well both qualitatively and quantitatively. The results illustrate that the optical emission is composed mainly of two stable-colour components, namely less variable thermal emission and highly variable synchrotron radiation. Furthermore, in most cases, the thermal component of BL Lacs is redder than that of synchrotron radiation, whereas the opposite is true for FSRQs.