We present a comprehensive, two-decade, multiwavelength variability study of the blazar PKS 1510 - 089, one of the most prominent and extensively monitored flat-spectrum radio quasars. 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. We find that all flux distributions prefer a double-log-normal fit. In case of the optical bands, this may be due to contributions from the accretion disk. The range of fluxes in a given band, as well as the fractional variability values are in-line with the expectations that high-energy parts of a given spectral component are more variable than low-energy parts. No obvious cross-correlations exist between the bands over the 20 years of observations. The X-ray and γ -ray spectra are variable, but do not show any trend with flux. These results are suggestive of different zones being active in the jet of PKS 1510 - 089 at any given time. In a previous paper, we used the same techniques to study the high-frequency-peaked BL Lac object PKS 2155 - 304. Both sources follow the aforementioned trend on the energy-dependent variability of the spectral components, as well as the lack of significant cross-correlations between the studied bands. While PKS 2155 - 304 exhibits a harder-when-brighter behaviour in its high-energy part of the synchrotron component, no such behaviour could be found in PKS 1510 - 089. Both sources show orphan flares, which can seemingly happen in any band. In summary, the long-term studies of these two sources reveal that the underlying physics is similar in these apparently different source classes, even though variability patterns keep changing and remain unpredictable.
Zacharias et al. (Thu,) studied this question.