Simultaneous BV R Monitoring of BL Lacertae Object S5 0716+714: Detection of Time Lags between Variations at Different Wavelengths and Study of Color Behavior

We present the results of our optical monitoring of the BL Lac object S5 0716+714 on seven nights in 2006 December. The monitoring was carried out simultaneously at three optical wavelengths with a novel photometric system. The object did not show strong internight variations, but displayed clear intranight variations. It tended to have larger variation amplitudes at shorter wavelengths. Cross correlation analyses revealed time lags between the intranight variations at different wavelengths. Mostly, the variations at short wavelengths lead those at longer wavelengths by up to about 35 minutes. This is the first time that the lags were detected between variations at different optical wavelengths for S5 0716+714 with high confidence levels. Furthermore, the object showed a strong bluer-when-brighter chromatism on both internight and intranight timescales. Subject headings: BL Lacertae objects: individual (S5 0716+714) — galaxies: active — galaxies: photometry