Quasi-simultaneous multicolour optical variability of S5 0716+714

Abstract

In order to investigate the physical process of blazar variability, S5 0716+714 was monitored quasi-simultaneously in V, R and I band from 2009 February to 2014 February. 6176 photometric observations on 42 nights were obtained and analysed. The F-test and analysis of variance test methods were used to detect variability for 83 intranight light curves, and a variability duty cycle of 83.9 per cent was derived. The variability amplitude in V band was slightly larger than that in R band, and a mild trend of having larger variability amplitude when source is getting fainter was shown for the intranight variations in both V and R bands. We found that the brightening phase of intranight variability was faster than fading process for both V and R band by analysing the distribution of the variability change rates calculated by linear fitting. A Kolmogorov–Smirnov test of change rates showed that the variability was time asymmetric. Quasi-simultaneous multicolour observations enabled us to obtain 2303 V − R colour indices. A strong bluer-when-brighter trend on intranight time-scales and a mild bluer-when-brighter colour variation on internight time-scales were derived by correlation analysis. The variability characteristics support the intrinsic shock-in-jet models and geometric effects, which indicates that the Doppler factor changes due to the change in the viewing angel of the relativistic jet.