Optical Monitoring of BL Lacertae Object S5 0716+714 with High Temporal Resolution

Optical monitoring of S5 0716+714 was performed with a 60/90 Schmidt telescope in 2003 November and December and 2004 January for studying the variability of the object on short timescales. Because of the high brightness of the source, we could carry out quasi-simultaneous measurements in three bands with a temporal resolution of about 20 minutes by using one single telescope. Intraday and intranight variations were observed, showing an overall change of ∼0.9 mag during the whole campaign. Two outbursts were recorded on Julian Dates 2,453,005 and 2,453,009. Minimum timescales of a few hours were derived from the light curves of individual nights, but they were different from night to night. A bluer-when-brighter chromatism was present when the object was showing a fast flare but was absent when it was in a relatively quiescent state. Our results are basically consistent with the shock-in-jet model and demonstrate that geometric effects can sometimes play an important role in the variability of blazars.