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Accès ouvert déclaré 2023 preprint

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

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Aims. We have performed the first broadband study of Mrk 421 from radio to TeV gamma rays with simultaneous measurements of theX-ray polarization from IXPE.Methods. The data were collected as part of an extensive multiwavelength campaign carried out between May and June 2022 using MAGIC,Fermi-LAT, NuSTAR, XMM-Newton, Swift, and several optical and radio telescopes to complement IXPE data.Results. 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 CrabNebula without intra-night variability. Throughout the campaign, the very high-energy (VHE) and X-ray emission are positively correlated ata 4σ significance level. The IXPE measurements reveal an X-ray polarization degree that is a factor of 2–5 higher than in the optical/radiobands; 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 duringa large fraction of the swing, the Swift-XRT monitoring reveals an X-ray flux increase with a clear spectral hardening. This suggests that flaresin 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, NuSTAR data reveal two contiguous spectral hysteresis loops in opposite directions (clockwise andcounterclockwise), implying important changes in the particle acceleration efficiency on approximately hour timescales.

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Les sujets associés

Astrophysics and Cosmic PhenomenaParticle Accelerators and Free-Electron LasersDark Matter and Cosmic Phenomena

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