Intraday multiband optical variability of BL Lacertae object S5 0716+714

Abstract

ABSTRACT We monitored the BL Lac object S5 0716+714 in four optical wavebands with four telescopes on 11 nights from 2012 to 2017. The object was active during most of those nights. It was brightest on 2012 November 7 with 11.720 mag in the I band and reached the faintest state on 2016 January 22 with 15.770 mag in the R band corresponding to ∼15.230 mag in the I band. Intraday variations in all bands were found on 6 d, while in partial bands on 3 d. Strong bluer-when-brighter chromatism was observed on 8 nights. Correlation analyses revealed interband time delays from 4 to 40 min in the intraday variability on 5 nights. Both hard and soft lags were observed in this object, indicating complicated emission process in the jet. The analysis on the interband time delays indicated that the radiative cooling may dominate the optical time lags in S5 0716+714 occasionally, but cannot be invoked as the sole explanation.