PKS 2155−304: long-term optical photometric monitoring and variability analysis

ABSTRACT Through the detailed study of the optical flux behaviour in blazars over time, it is possible to infer the conditions responsible for their observed emission. PKS 2155$-$304, a BL Lac object detected from radio to TeV energies, is among the brightest blazars in the Southern hemisphere. We present optical monitoring spanning over two decades using telescopes at Complejo Astronómico El Leoncito and Estación Astrofísica de Bosque Alegre, Argentina. Differential light curves in the B, V, R, and I bands reveal significant variability on weekly and longer time-scales, with occasional changes on sub-four-hour scales. The optical spectral index remained negative, consistent with non-thermal emission, and hardened over the past 9 yr. Evidence for quasi-periodic behaviour on 20–30 d time-scales was found, while correlations with X-ray fluxes were weak, suggesting distinct emission components in the two bands. These results highlight the pronounced optical variability of PKS 2155$-$304 and provide insight into its multiband emission mechanisms.