Multi-colour optical variability of the blazar 3C 454.3 in 2007-2010

Context. We present our optical multiband (B and R) observations of the blazar 3C 454.3, which were acquired over 30 nights from August 2007 to December 2010. Aims. We analyse our data to study the optical emission variability properties of 3C 454.3. Methods. The observations were carried out using both the 1 m EOS Telescope (EST) and the 0.8 m Tsinghua-NAOC Telescope (TNT) simultaneously in most nights at the Xinglong Observatory of National Astronomical Observatories, Chinese Academy of Sciences (NAOC). Both telescopes are equipped with the same type of Princeton Instrument CCD and standard Johnson-Cousin UBVRI filters. To reach a similar signal-to-noise ratio (S/N) in different bands with the same exposure time, observations using the larger diameter EST were carried out with B-band filter, while those of the smaller diameter TNT were carried out with R-band filter. Any time differences between the two telescopes were corrected using GPS. The typical sampling rate was one data point per minute, which is the highest time-resolution so far. Results. A multi-peak outburst was observed during November-December 2007, in addition to a later outburst during October 27-November 1, 2010. The brightest magnitudes for 3C 454.3 during our observations were B = 13.77 mag and R = 12.69 mag at JD = 2 454 436.0952 (December 1, 2007). One of the most striking events was detected on December 1, 2007, when a flux increase of about 0.67 mag occurred in 3.2 h. A colour analysis shows that the colour variability processes are apparently complex. A bluer-when-brighter trend, a redder-when-brighter trend (generally in faint states), and no significant colour variance were all detected during the outburst of November-December 2007. Furthermore, the kind of "saturation" effect in the B - R trend is not visible in the bright state (R < 14) during August 12-14, 2007. These trends may be due to the interactive contribution from the accretion disc and the jet components. Meanwhile, intra-night variability in B and R bands was found to be correlated, without any time delay exceeding one minute.