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We report on the correlation between the flux, color and polarizationvariations on time scales of days--months in blazars, and discuss theiruniversal aspects. We performed monitoring of 42 blazars in the optical andnear-infrared bands from 2008 to 2010 using TRISPEC attached to the "Kanata"1.5-m telescope. We found that 28 blazars exhibited "bluer-when-brighter"trends in their whole or a part of time-series data sets. This corresponds to88% of objects that were observed for >10 days. Thus, our observationunambiguously confirmed that the "bluer-when-brighter" trend is common in theemission from blazar jets. This trend was apparently generated by a variationcomponent with a constant and relatively blue color and an underlying redcomponent. Prominent short-term flares on time scales of days--weeks tended toexhibit a spectral hysteresis; their rising phases were bluer than their decayphases around the flare maxima. In contrast to the strong flux--colorcorrelation, the correlation of the flux and polarization degree was relativelyweak; only 10 objects showed significant positive correlations. Rotations ofpolarization were detected only in three objects: PKS 1510-089, 3C 454.3, andPKS 1749+096, and possibly in S5 0716+714. We also investigated the dependenceof the degree of variability on the luminosity and the synchrotron peakfrequency, \nu_peak. As a result, we found that lower luminosity and higher\nu_peak objects had smaller variations in their amplitudes both in the flux,color, and polarization degree. Our observation suggests the presence ofseveral distinct emitting sources, which have different variation time-scales,colors, and polarizations. We propose that the energy injection by, forexample, internal shocks in relativistic shells is a major factor for blazarvariations on time scales of both days and months.

publications of the astronomical society of japan

Photopolarimetric Monitoring of Blazars in the Optical and Near-Infrared  Bands with the Kanata Telescope. I. Correlations between Flux, Color, and  Polarization