tokens: This work is supported by the National Natural Science Foundation of China under grant Nos 11203016 and 11143012. This work is partly supported by the Young Scholars Program of Shandong University, Weihai. We are also grateful for data taken and assembled by the WEBT collaboration and stored in the WEBT archive at the Osservatorio Astrofisico di Torino-INAF ( http://www.oato.inaf.it/blazars/webt/ ) and data from the Steward Observatory spectropolarimetric monitoring project supported by Fermi Guest Investigator grants NNX08AW56G, NNX09AU10G and NNX12AO93G, as well as data from the variable star observations from the AAVSO International Database contributed by observers worldwide.
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