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

TL;DR

This work analyzes two decades of Swift-UVOT and Swift-XRT data for Mrk 421 and Mrk 501 to probe long-term variability at synchrotron energies. Using structure functions, Lomb-Scargle periodograms, and cross-correlation methods, the authors find energy-dependent variability, lognormal flux distributions that vary by band, and a general harder-when-brighter trend in X-ray spectra described by a log-parabolic model. No persistent optical/UV–X-ray correlation emerges, reinforcing a multi-zone emission framework; a tentative QPO near $P'\approx 390$ days in Mrk 501 is discussed but requires further validation. Overall, the results constrain jet emission processes and the disc–jet connection in these nearby HBLs, highlighting the limitations of one-zone models for these systems.

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$σ$) 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.

Figures (8)

• Figure 1: Multiwavelength light curves of Mrk 421 UV, Mrk 421 X-ray, Mrk 501 optical, Mrk 501 UV, Mrk 501 X-ray. The legend shows the Swift telescope and filter used to collect the data, as well as the number of observations in each band. Each data point represents one observation. We present the full dynamic range of the data, in linear scaling, and observe higher variability at higher frequencies.
• Figure 2: Flux distributions for, from left to right, Mrk 421 UV, Mrk 421 X-ray, Mrk 501 UV, and Mrk 501 X-ray. The skewness parameter, $\gamma$, a metric of lognormality, is shown in the legend and increases with photon energy.
• Figure 3: X-ray log parabola fit parameters, $\alpha$ and $\beta$, as a function of flux. Mrk 421 (top) and Mrk 501 (bottom) follow the harder-when-brighter trend, with no trend in curvature.
• Figure 4: Structure functions for Mrk 421 UV (far left), Mrk 421 X-ray (centre left), Mrk 501 UV (centre right), and Mrk 501 X-ray (far right). After 2000 days these are badly affected by systematics, so peaks right of this value should not be trusted. Only Mrk 421 X-ray appears to reach saturation.
• Figure 5: Lomb-Scargle periodograms for Mrk 421 UV (top left), Mrk 421 X-ray (top right), Mrk 501 UV (bottom left), and Mrk 501 X-ray (bottom right). Only the latter contains a peak above the $3\sigma$ significance level, which falls at around 400 days.