Simultaneous optical monitoring of BL Lacertae object S5 0716+714 with
  high temporal resolution
  We have monitored the BL Lacertae object S5 0716+714 simultaneously in the B,
R and I bands on three nights in November 2014. The average time resolution is
quite high (73s, 34s, 58s 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 color behavior was found. An clear
loop path can be seen on the color-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 is approximately 1.5 minutes 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.
