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Abstract X-ray polarization is a unique new probe of the particle acceleration in astrophysical jets made possible through the Imaging X-ray Polarimetry Explorer. Here we report on the first dense X-ray polarization monitoring campaign on the blazar Mrk 421. Our observations were accompanied by an even denser radio and optical polarization campaign. We find significant short-timescale variability in both X-ray polarization degree and angle, including an ∼90° angle rotation about the jet axis. We attribute this to random variations of the magnetic field, consistent with the presence of turbulence but also unlikely to be explained by turbulence alone. At the same time, the degree of lower-energy polarization is significantly lower and shows no more than mild variability. Our campaign provides further evidence for a scenario in which energy-stratified shock-acceleration of relativistic electrons, combined with a turbulent magnetic field, is responsible for optical to X-ray synchrotron emission in blazar jets.
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Linear Polarization Signatures of Particle Acceleration in High-Synchrotron-Peak Blazars
Blazars whose synchrotron spectral energy distribution (SED) peaks at X-ray energies need to accelerate electrons to energies in the >100 GeV range in relativistic plasma jets at distances of parsecs from the central engine. Compton scattering by the same electrons can explain high luminosities at very high photon energies (>100 GeV) from the same objects. Turbulence combined with a standing conical shock can accomplish this. Such a scenario can also qualitatively explain the level and variability of linear polarization observed at optical frequencies in these objects. Multi-wavelength polarization measurements, including those at X-ray energies by the Imaging X-ray Polarimetry Explorer (IXPE), find that the degree of polarization is several times higher at X-ray than at optical wavelengths, in general agreement with the turbulence-plus-shock picture. Some detailed properties of the observed polarization can be naturally explained by this scenario, while others pose challenges that may require modifications to the model.
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- Award ID(s):
- 2108622
- PAR ID:
- 10436060
- Date Published:
- Journal Name:
- Universe
- Volume:
- 8
- Issue:
- 12
- ISSN:
- 2218-1997
- Page Range / eLocation ID:
- 644
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3390/universe8120644
