Not so Swift: 20 years of multiwavelength observations of Mrk 421 and Mrk 501

TL;DR

This study analyzes 20 years of multiwavelength data from Swift telescopes to understand long-term variability in blazars Mrk 421 and Mrk 501, revealing energy-dependent variability, flux distribution trends, and potential quasi-periodicity.

Contribution

It provides the longest multiwavelength temporal analysis of these blazars, highlighting energy-dependent variability and potential quasi-periodic signals in X-ray data.

Findings

Variability is energy dependent across wavelengths.

Flux distributions show lognormality.

Hints of quasi-periodicity in Mrk 501's X-ray data.

Abstract

Aims. The blazars Mrk 421 and Mrk 501 have shown multiwavelength 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 optical, UV, and X-ray, in order to assess current models of these objects and their processes. Methods. 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 vs 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 in 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 of Mrk 501 (host frame time scale $\sim390$ days, >3$\sigma$) but not in the UV or X-ray of Mrk 421, or in the UV of Mrk 501. No correlation at any time lag was found between the optical/UV and X-ray bands in either source.