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 0.457 0.014% var =  , 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 (F pp ) in each band is below 1.0%, and the F pp 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 10Using 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 30.2 mJy V > . The weak of the correlation between B-and I-band and the spectrum analysis indicate that the optical radiation consists of two variable components.

Abstract:We present long-term optical multi-band photometric monitoring of the blazar OJ 287 from 6 March 2010 to 3 April 2016, with high temporal resolution in the VRI-bands. The flux variations and colour-magnitude variations on long and short timescales were investigated to understand the emission mechanisms. In our observation, the major outbursts occurred in January 2016, as predicted by the binary pair of black holes model for OJ 287, with F var of 1.3∼2.1%, and variability amplitude (Amp) of 5.8∼9.0%. The intra-night variability (IDV) durations were from 18.5 to 51.3 min, and the minimal variability timescale was about 4.7 min. The colour-magnitude variation showed a weak positive correlation on the long timescale with Pearson's r = 0.450, while a negative correlation was found on intra-night timescales. We briefly discuss the possible physical mechanisms that are most likely to be responsible for the observed flux and colour-magnitude correlation variability.