Study on Variability and Spectral Properties of Blazar 3C 273 with Long-term Multi-band Optical Monitoring from 2006 to 2015

We present long-term optical multi-band photometric monitoring of blazar 3C 273, from 2006 May 19 to 2015 March 31 with high temporal resolution in the BVRI bands. The source is in a steady state and showed very small variability, with the values of the fractional variability amplitude of F var = 0.457 0.014 % , 0.391 0.012 % , 0.264 0.043 % and 0.460 0.014 % in B, V, R and I, respectively. The intra-night point-to-point fractional variability (Fpp) in each band is below 1.0%, and the Fpp variation amplitude increase from the B-band to the I-band. We find a variability with the timescale of 5.8 2.9 minutes in the I-band on 2009 March 11. This fast variability requires the comoving magnetic field strength in the jet above 18 G with a Doppler factor δ D ∼ 10 . Using the discrete correlation function (DCF), the B- and I-band light curves are examined for correlation on whole campaign. Low significance (∼99.73 percent confidence) correlations with the I-band lags the B-band variations are observed. The spectral behaviors in the different variability episodes are studied. "Bluer-when-brighter" spectral behavior is presented for the whole campaign, while there is an opposite tendency when F V > 30.2 mJy . The weak of the correlation between B- and I-band and the spectrum analysis indicate that the optical radiation consists of two variable components.