Multi-color optical monitoring of ten blazars from 2005 to 2011
  We carried out multi-color optical monitoring of a sample of ten blazars from
2005 to 2011. The sample contains 3 LBLs, 2 IBLs, 4 HBLs, and 1 FSRQ. Our
monitoring focused on the long-term variability and the sample included nine BL
Lac objects and one flat-spectrum radio quasar. A total number of 14799 data
points were collected. This is one of the largest optical database for a sample
of ten blazars. All objects showed significant variability except OT 546.
Because of the low sampling on each single night, only BL Lacertae was observed
to have intra-day variability on 2006 November 6. Most BL Lac objects showed a
bluer-when-brighter chromatism, while the flat-spectrum radio quasar, 3C 454.3,
displayed a redder-when-brighter trend. The BWB color behaviors of most BL Lacs
can be at least partly interpreted by the fact of increasing variation
amplitude with increasing frequency observed in these objects. The average
spectral index of LBLs is around 1.5, as expected from the model dominated by
Synchrotron Self-Compton (SSC) loss. The optical emission of HBL is probably
contaminated by the thermal emission from the host galaxies. Correlation
analysis did not reveal any time delay between variations at different
wavelengths.
