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

TL;DR

This study reports the first detection of X-ray polarization in the jet of Mrk 421 using IXPE, revealing insights into particle acceleration and magnetic field structure in blazar jets during an average flux state.

Contribution

It provides the first measurement of X-ray polarization in Mrk 421, demonstrating the utility of IXPE in probing jet emission mechanisms and magnetic field configurations in high-frequency-peaked BL Lac objects.

Findings

X-ray polarization degree is 15±2%, higher than optical/infrared/millimeter wavelengths.

X-ray flux increased by a factor of 2.2 with no significant polarization variability.

Results support a shock as the X-ray emission site and an energy-stratified electron population.

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_{\rm x}=15\pm$2\% and electric-vector position angle $\Psi_{\rm x}=35^\circ\pm4^\circ$) 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 $\Pi_{\rm x}$ 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.