Multi-color optical monitoring of the quasar 3C 273 from 2005 to 2016

Abstract

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 $△I=0^{\mathrm{m}}.67$, $△R=0^{\mathrm{m}}.72$, $△V=0^{\mathrm{m}}.68$, and $△(V-R)=0^{\mathrm{m}}.25$ 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 $P=3918\pm 1112$ days. The possible explanations for the observed variability, BWB chromatic trend and periodicity are discussed.

63 pages, 11 figures, 6 tables. Accepted for publication in ApJS