Multicolor Optical Monitoring of the Blazar S5 0716+714 from 2017 to
  2019
  We continuously monitored the blazar S5 0716+714 in the optical $g$, $r$ and
$i$ bands from Nov. 10, 2017 to Jun. 06, 2019. The total number of observations
is 201 nights including 26973 data points. This is a very large
quasi-simultaneous multicolor sample for the blazar. The average time spans and
time resolutions are 3.4 hours and 2.9 minutes per night, respectively. During
the period of observations, the target source in the $r$ band brightens from
$14^{\rm m}.16$ to $12^{\rm m}.29$ together with five prominent sub-flares, and
then first becomes fainter to $14^{\rm m}.76$ and again brightens to $12^{\rm
m}.94$ with seven prominent sub-flares. For the long-term variations, we find a
strong flatter when brighter (FWB) trend at a low flux state and then a weak
FWB trend at a higher flux state. A weak FWB trend at a low flux state and then
a strong FWB trend at a higher flux state are also reported. Most of sub-flares
show the strong FWB trends, except for two flares with a weak FWB trend. The
particle acceleration and cooling mechanisms together with the superposition of
different FWB-slopes from sub-flares are likely to explain the optical color
behaviours. A scenario of bent jet is discussed.
