First characterization of the emission behavior of Mrk 421 from radio to very high-energy gamma rays with simultaneous X-ray polarization measurements

Abstract

<p><strong>Aims.</strong> <br></p><p>We have performed the first broadband study of Mrk 421 from radio to TeV gamma rays with simultaneous measurements of the X-ray polarization from IXPE. <br></p><p><strong>Methods.</strong> <br></p><p>The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC, <em>Fermi</em>-LAT, <em>NuSTAR</em>, <em>XMM-Newton</em>, <em>Swift</em>, and several optical and radio telescopes to complement IXPE data. <br></p><p><strong>Results. </strong><br></p><p>During the IXPE exposures, the measured 0.2-1 TeV flux was close to the quiescent state and ranged from 25% to 50% of the Crab Nebula without intra-night variability. Throughout the campaign, the very high-energy (VHE) and X-ray emission are positively correlated at a 4σ significance level. The IXPE measurements reveal an X-ray polarization degree that is a factor of 2-5 higher than in the optical/radio bands; that implies an energy-stratified jet in which the VHE photons are emitted co-spatially with the X-rays, in the vicinity of a shock front. The June 2022 observations exhibit a rotation of the X-ray polarization angle. Despite no simultaneous VHE coverage being available during a large fraction of the swing, the <em>Swift</em>-XRT monitoring reveals an X-ray flux increase with a clear spectral hardening. This suggests that flares in high synchrotron peaked blazars can be accompanied by a polarization angle rotation, as observed in some flat spectrum radio quasars. Finally, during the polarization angle rotation, <em>NuSTAR</em> data reveal two contiguous spectral hysteresis loops in opposite directions (clockwise and counterclockwise), implying important changes in the particle acceleration efficiency on approximately hour timescales.<br></p>