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A spectral variation accompanied with flux variability is a commonly-observed phenomenon for blazars. In order to further investigate the optical spectral feature of blazars, we have collected the long-term optical V and R band data of 27 blazars (14 BL Lacs and 13 FSRQs), and calculated their optical spectral indices. The results show that the spectral indices vary with respect to the 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 be stable. In other words, the source becomes bluer (or redder) and then gradually stabilizes when it brightens, which are briefly named the bluer-stable-when-brighter (BSWB) and redder-stable-when-brighter (RSWB) behaviors, respectively. Thirteen of the 14 BL Lacs show the BSWB behavior, with an exception of AO 0235+164. On the contrary, most of FSRQs (10 out of 13) exhibit the RSWB trend. It is confirmed that blazars follow the two universal optical spectral behaviors, namely, BSWB and RSWB. The model of two constant-spectral-index components can well explain the optical spectral features qualitatively and quantitatively. The results illustrate that the optical emission are mainly composed of two stable-color components, i.e., less variable thermal emission and high variable synchrotron radiation. And in most cases, the thermal component of BL Lacs is redder than that of synchrotron radiation, whereas FSRQs are the opposite., Comment: 9 pages, 14 figures, accepted by MNRAS

The long-term optical, X-ray and $\gamma$-ray data of blazar 3C 279 have been compiled from $Swift$-XRT, $RXTE$ PCA, $Fermi$-LAT, SMARTS and literature. The source exhibits strong variability on long time scales. Since 1980s to now, the optical $R$ band light curve spans above 32 yr, and a possible 5.6-yr-long quasi-periodic variation component has been found in it. The optical spectral behavior has been investigated. In the optical band, the mean spectral index is -1.71. The source exhibits an obvious special spectral behavior. In the low state, the source shows a clear bluer-when-brighter behavior in a sense that the optical spectrum turns harder (flatter) when the brightness increases. While in the high state, the optical spectrum is stable, that means the source spectral index does not vary with the brightness. The correlation analysis has been performed among optical, X-ray and $\gamma$-ray energy bands. The result indicates that the variations of $\gamma$-ray and X-ray bands are well correlated without time delay on the time scale of days, and their variations exhibit weak correlations with those of optical band. The variations, especial outbursts, are simultaneous, but the magnitude of variations is disproportionate. The detailed analysis reveals that the main outbursts exhibit strong correlations in different $\gamma$-ray, X-ray and optical bands., Comment: 17 pages, 13 figures. Accepted by Research in Astron. Astrophys.(RAA)

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., Comment: 11 pages, 5 figures. Accepted by RAA (Res. Astron. Astrophys.)

PKS 2155-304 is a well studied BL Lac object in the southern sky. The historical optical data during different period have been collected and compiled. Light curves with a time span of 35 years have been constructed. The R-band light curve has been analyzed by means of three methods: epoch folding method, Jurkevich method and discrete correlation function (DCF) method. It is derived that there is an evident periodic component of 317 days (i.e. 0.87 yr) superposed on a long-term trend with large-amplitude variation in the light curve. The variability of this source is accompanied by a slight color variation, and the brightness and color index are correlated with each other. On the long time-scale, PKS 2155-304 exhibits a tendency of bluer-when-brighter, which means the spectrum becomes flatter when the source brightens., Comment: 9 pages, 4 figures. Accepted for publication in RAA (Research in Astronomy and Astrophysics)