2016MNRAS.456.3168M

We have monitored the BL Lacertae object S5 0716+714 simultaneously in the B, R and I bands on three nights in 2014 November. The average time resolution is quite high (73, 34 and 58 s for the filters B, R and I), which can help us trace the profile of the variation and search for the short inter-band time delay. Intra-day variability was about 0.1 mag on the first two nights and more than 0.3 mag on the third. A bluer-when-brighter colour behaviour was found. A clear loop path can be seen on the colour-magnitude diagram of the third night, revealing possible time delays between variations at high and low energies. It is the first time that the intra-day spectral hysteresis loop has been found so obviously in the optical band. We used the interpolated cross-correlation function method to further confirm the time delay and calculated the values of lag between light curves at different wavelengths on each night. On the third night, variations in the R and B bands are approximately 1.5 min lagging behind the I band. Such optical time delay is probably due to the interplay of different processes of electrons in the jet of the blazar.