Not so Swift: 20 years of multi-wavelength observations of Mrk 421 and Mrk 501

Aims. The blazars Mrk 421 and Mrk 501 have shown multi-wavelength variability on all observed timescales, and have been well studied at high energies on short timescales. We aim to characterise the long-term temporal behaviour of these blazars at synchrotron energies, namely in the optical, UV, and X-ray, in order to assess current models of these objects and their processes. Methods. Including amongst the longest light curves ever studied for these sources, we investigated 20 years of data (2005–2025) from the Swift-UVOT and Swift-XRT telescopes. We examined spectral models, fractional variabilities, flux distributions, and X-ray photon index versus flux relations, as well as carrying out in-depth time series analysis using structure functions, Lomb-Scargle periodograms, and discrete correlation functions. Results. Mrk 421 and Mrk 501 both showed intriguing variability at all studied wavelengths; this variability has been found to be energy-dependent, as has the trend of lognormality in flux distributions. X-ray photon indices fluctuated greatly throughout the entire period, showing an overall harder-when-brighter trend. Hints of a quasi-periodicity have been found in the X-ray data of Mrk 501 (host frame timescale of ∼390 days, > 3 σ ) but not in the UV or X-ray data of Mrk 421, or in the UV data of Mrk 501. No correlation at any time lag was found between the optical/UV and X-ray bands in either source.