Multi-color optical monitoring of the quasar 3C 273 from 2005 to 2016
  We have monitored the quasar 3C 273 in optical $V$, $R$ and $I$ bands from
2005 to 2016. Intraday variability (IDV) is detected on seven nights. The
variability amplitudes for most of nights are less than 10\% and four nights
more than 20\%. When considering the nights with time spans $>4$ hours, the
value of duty cycle (DC) is 14.17 per cent. Over the twelve years, the overall
magnitude and color index variabilities are $\bigtriangleup I=0^{\rm m}.67$,
$\bigtriangleup R=0^{\rm m}.72$, $\bigtriangleup V=0^{\rm m}.68$, and
$\bigtriangleup (V-R)=0^{\rm m}.25$ respectively. The largest clear IDV has an
amplitude of 42% over just 5.8 minutes and the weakest detected IDV is 5.4%
over 175 minutes. The BWB (bluer when brighter) chromatic trend is dominant for
3C 273 and appears at different flux levels on intraday timescales. The BWB
trend exists for short-term timescales and intermediate-term timescales but
different timescales have different correlations. There is no BWB trend for our
whole time-series data sets. A significant anti-correlation between BWB trend
and length of timescales is found. Combining with $V$-band data from previous
works, we find a possible quasi-periodicity of $P=3918\pm1112$ days. The
possible explanations for the observed variability, BWB chromatic trend and
periodicity are discussed.
