Multicolour optical and near-infrared variability of the blazar PKS 2155-304 on diverse time-scales

Blazar PKS 2155-304 is one of the sources that has been well monitored under the SMARTS (Small Moderate Aperture Research Telescope System) project. SMARTS light curves in the BVRJK bands and spectral indices connecting the B with the J band of PKS 2155-304 from 2008 to 2015 are presented at near-nightly cadence, and divided into a number of segments without long observational gaps for comparison investigations on shorter time-scales. The optical and near-infrared (NIR) variations are simultaneous, with time lags of no longer than a couple of days, which suggests a temporal coherence of the corresponding jet-emitting regions. Long-and short-term optical-NIR variability shows that optical bands are fainter and vary less than the IR bands, while fractional variability amplitudes show no clear trend on long-term time-scales and in segments of light curves, probably owing to effects from stochastic processes. The prominent flares presented in the optical-NIR light curves tend to show an asymmetric feature with different rise and decay time-scales. The spectral indices varied on diverse time-scales, and the source showed no clear trend on long-term time-scales, an occasional appearance of tendencies of flatter-when-brighter on short-term time-scales, and an independent clockwise evolving loop pattern, suggesting a complex spectral evolution with the flux.