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

<jats:p><jats:italic>Aims.</jats:italic> 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.</jats:p>
<jats:p><jats:italic>Methods.</jats:italic> The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC, <jats:italic>Fermi</jats:italic>-LAT, <jats:italic>NuSTAR</jats:italic>, <jats:italic>XMM-Newton</jats:italic>, <jats:italic>Swift</jats:italic>, and several optical and radio telescopes to complement IXPE data.</jats:p>
<jats:p><jats:italic>Results.</jats:italic> 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 4<jats:italic>σ</jats:italic> 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 <jats:italic>Swift</jats:italic>-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, <jats:italic>NuSTAR</jats:italic> 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.</jats:p>