The X-ray Polarization View of Mrk~421 in an Average Flux State as Observed by the Imaging X-ray Polarimetry Explorer

Abstract

Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multi-wavelength polarization properties of blazars. The recent launch of the Imaging X-ray Polarimetry Explorer (IXPE) enables us, for the first time, to use polarization in the X-ray band (2-8 keV) to probe the properties of the jet synchrotron emission in high-frequency-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree ${\Pi }_{\mathrm{x}}=15\pm$2\% and electric-vector position angle ${\Psi }_{\mathrm{x}}=35^{\circ }\pm 4^{\circ }$) from the jet of the HSP Mrk~421 in an average X-ray flux state. At the same time, the degree of polarization at optical, infrared, and millimeter wavelengths was found to be lower by at least a factor of 3. During the IXPE pointing, the X-ray flux of the source increased by a factor of 2.2, while the polarization behavior was consistent with no variability. The higher level of ${\Pi }_{\mathrm{x}}$ compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock as the most likely X-ray emission site in the jet of Mrk 421 during the observation. The multiwavelength polarization properties are consistent with an energy-stratified electron population, where the particles emitting at longer wavelengths are located farther from the acceleration site, where they experience a more disordered magnetic field.

accepted for ApJ Letters

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Cited by in corpus (22)

• Uncovering the geometry of the hot X-ray corona in the Seyfert galaxy NGC4151 with IXPE
• X-ray Polarization Observations of BL Lacertae
• Magnetic Field Properties inside the Jet of Mrk 421: Multiwavelength Polarimetry Including the Imaging X-ray Polarimetry Explorer
• IXPE observation of PKS 2155-304 reveals the most highly polarized blazar
• Detection of X-ray Polarization from the Blazar 1ES 1959+650 with the Imaging X-ray Polarimetry Explorer
• Insights into the broad-band emission of the TeV blazar Mrk 501 during the first X-ray polarization measurements
• The variability of blazars throughout the electromagnetic spectrum
• The polarization of the synchrotron radiation from a recollimated jet: application to high-energy BL Lacs
• Revisiting the proton synchrotron radiation in blazar jets: Possible contributions from X-ray to $\gamma$-ray bands
• IXPE Observation of the Low-Synchrotron Peaked Blazar S4 0954+65 During An Optical-X-ray Flare
• Multifrequency polarimetry of High-Synchrotron Peaked blazars probes the shape of their jets
• X-ray Polarization of the High-Synchrotron-Peak BL Lacertae Object 1ES 1959+650 during Intermediate and High X-ray Flux States
• Axion-like Particle Effects on Photon Polarization in High-Energy Astrophysics
• Insights from the first flaring activity of a high-synchrotron-peaked blazar with X-ray polarization and VHE gamma rays
• Insights into the Long-Term Flaring Events of Blazar PKS 0805-07: A Multi-Wavelength Analysis over the 2009-2023 Period
• Optical Spectropolarimetric Variability Properties in Blazars PKS 0637-75 and PKS 1510-089
• Quantitative comparisons of VHE gamma-ray blazar flares with relativistic reconnection models
• Testing particle acceleration in blazar jets with continuous high-cadence optical polarization observations
• Spectral Energy Distribution Modeling of BL Lacertae During a Large Submillimeter Outburst and Low X-Ray Polarization State
• The Method of Searching for Rotations of the Polarization Position Angle of Quasars
• Detection of Compton scattering in the jet of 3C 84
• Fast Optical Variability of the TeV Blazar PKS 1725+123 Observed by SVOM-VT and Insights from Multi-wavelength Follow-up Observations