The optical spectral features of 27 <i>Fermi</i> blazars

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