Multicolor photopolarimetric observation of 15min variability in S 5 0716 + 714

We present the result of near-infrared and optical observations of the BL Lac object S5 0716+714 carried out by the KANATA telescope (Uemura et al. 2008 in these proceedings). S5 0716+714 has both a long term violent variability and a short-term variability. The shortest time-scale variability, so-called microvariability, provides a clue of the minimum size of the emitting region. Here, we report the detection of 15-min variability in S5 0716+714, which is one of the shortest time-scale in the optical and near-infrared variations observed in blazars. The detected microvariation had a small amplitude of 0.06±0.01 mag in V band and a blue color of ∆(V − J) = −0.03±0.01. The microvariation has a feature of “bluer-when-brighter” that S5 0716+714 has in the variation time-scale of several hours. We furthermore obtained the temporal variation of polarization associated with the microvariation. We revealed that the microvariation had a specific polarization component whose polarization degree was higher than that of the overall trend. These results suggest that the microvariability originated from a small and local region where the magnetic field is aligned.