20 Years of monitoring: PKS 2155-304 and PKS 1510-089 in the eyes of Swift and Fermi. I. The case of PKS 2155-304

We present a comprehensive 20-year multiwavelength variability study of the blazar PKS 2155 - 304, one of the most luminous and extensively monitored high-frequency-peaked BL Lac objects in the southern hemisphere. Using Fermi -LAT γ -ray data together with Swift -XRT and UVOT observations spanning 2005–2024, we trace the long-term evolution of its flux, interband correlations, and spectral behaviour across the optical, X-ray, and γ -ray bands. While the HE γ -ray and X-ray flux distributions are compatible with log-normality, the optical band prefers a double-log-normal fit. The latter matches well the baseline flux change that took place in the optical range around 2009, and which is clearly visible in the flux distribution. While interband flux–flux correlations are found, no stable temporal lags emerge. This implies varying correlation patterns between epochs. The X-ray emission displays a robust harder-when-brighter trend, however with epoch-dependent slopes, while the γ -ray spectra show only mild flux dependence. The fractional variability increases systematically with energy within a given radiation component. No direct correlation of the year-wise fractional variability with the corresponding average flux could be found. Interestingly, a pronounced X-ray spectral upturn, detected during a low state in 2012, points to an additional radiative component. As the connection from this upturn to the γ -ray spectrum is not smooth, it probably is not the onset of the inverse-Compton component, but more likely points either to a hadronic contribution or an additional spatially-separate emission zone. These findings reveal the complexity of variability patterns in PKS 2155 - 304 and the non-uniform nature of its particle acceleration and emission processes.