Multicolor Optical Monitoring of the Blazar S5 0716+714 from 2017 to 2019

Abstract

We continuously monitored the blazar S5 0716+714 in the optical g , r , and i bands from 2017 November 10 to 2019 June 6. The total number of observations is 201 nights including 26,973 data points. This is a very large quasi-simultaneous multicolor sample for the blazar. The average time spans and time resolutions are 3.4 hr and 2.9 minutes per night, respectively. During the period of observations, the target source in the r band brightens from 14.ͫ16 to 12.ͫ29 together with five prominent subflares, and then becomes fainter to 14.ͫ76, and again brightens to 12.ͫ94 with seven prominent subflares. 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 subflares show strong FWB trends, except for two flares with a weak FWB trend. The particle acceleration and cooling mechanisms together with the superposition of the different FWB slopes from the subflares likely explain the optical color behaviors. A scenario of bent jet is discussed.