Dissecting the long-term emission behaviour of the BL Lac object Mrk 421

TL;DR

This study presents a comprehensive long-term multi-wavelength analysis of blazar Mrk 421, revealing complex variability, polarization behavior, and spectral shifts, supporting jet models with multiple emitting regions and changing orientations.

Contribution

It provides the first detailed multi-wavelength, spectropolarimetric, and spectral energy distribution analysis of Mrk 421 over several years, highlighting variability patterns and jet model implications.

Findings

Gamma-ray flux correlates with optical flux from 2012 onward.

Wavelength-dependent polarization degree indicates host galaxy influence.

Spectral energy distributions show synchrotron peak shifts up to a factor of 50.

Abstract

We report on long-term multi-wavelength monitoring of blazar Mrk 421 by the GASP-WEBT collaboration and Steward Observatory, and by the Swift and Fermi satellites. We study the source behaviour in the period 2007-2015, characterized by several extreme flares. The ratio between the optical, X-ray, and \gamma-ray fluxes is very variable. The \gamma-ray flux variations show fair correlation with the optical ones starting from 2012. We analyse spectropolarimetric data and find wavelength-dependence of the polarisation degree (P), which is compatible with the presence of the host galaxy, and no wavelength-dependence of the electric vector polarisation angle (EVPA). Optical polarimetry shows a lack of simple correlation between P and flux and wide rotations of the EVPA. We build broad-band spectral energy distributions with simultaneous near-infrared and optical data from the GASP-WEBT and UV and X-ray data from the Swift satellite. They show strong variability in both flux and X-ray spectral shape and suggest a shift of the synchrotron peak up to a factor $\sim 50$ in frequency. The interpretation of the flux and spectral variability is compatible with jet models including at least two emitting regions that can change their orientation with respect to the line of sight.