2026A&A...706A.366T

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, > 3σ) 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.