2009ApJS..185..511P

We monitored the BL Lac object S5 0716+714 in the optical band during 2008 October and December and 2009 February with a best temporal resolution of about 5 minutes in the BVRI bands. Four fast flares were observed with amplitudes ranging from 0.3 to 0.75 mag. The source remained active during the whole monitoring campaign, showing microvariability in all days except for one. The overall variability amplitudes are ΔB ∼ 0^m.89, ΔV ∼ 0^m.80, ΔR ∼ 0^m.73, and ΔI∼ 0^m.51. Typical timescales of microvariability range from 2 to 8 hr. The overall V - R color index ranges from 0.37 to 0.59. Strong bluer-when-brighter chromatism was found on internight timescales. However, a different spectral behavior was found on intranight timescales. A possible time lag of ∼11 minutes between B and I bands was found on one night. The shock-in-jet model and geometric effects can be applied to explain the source's intranight behavior.