Kavli Affiliate: Herman L. Marshall
| First 5 Authors: Dawoon E. Kim, Laura Di Gesu, Ioannis Liodakis, Alan P. Marscher, Svetlana G. Jorstad
| Summary:
We conducted a polarimetry campaign from radio to X-ray wavelengths of the
high-synchrotron-peak (HSP) blazar Mrk 421, including Imaging X-ray Polarimetry
Explorer (IXPE) measurements on 2022 December 6-8. We detected X-ray
polarization of Mrk 421 with a degree of $Pi_{rm X}$=14$pm$1$%$ and an
electric-vector position angle $psi_{rm X}$=107$pm$3$^{circ}$ in the 2-8
keV band. From the time variability analysis, we find a significant episodic
variation in $psi_{rm X}$. During 7 months from the first IXPE pointing of
Mrk 421 in 2022 May, $psi_{rm X}$ varied across the range of 0$^{circ}$ to
180$^{circ}$, while $Pi_{rm X}$ maintained similar values within
$sim$10-15$%$. Furthermore, a swing in $psi_{rm X}$ in 2022 June was
accompanied by simultaneous spectral variations. The results of the
multiwavelength polarimetry show that the X-ray polarization degree was
generally $sim$2-3 times greater than that at longer wavelengths, while the
polarization angle fluctuated. Additionally, based on radio, infrared, and
optical polarimetry, we find that rotation of $psi$ occurred in the opposite
direction with respect to the rotation of $psi_{rm X}$ over longer timescales
at similar epochs. The polarization behavior observed across multiple
wavelengths is consistent with previous IXPE findings for HSP blazars. This
result favors the energy-stratified shock model developed to explain variable
emission in relativistic jets. The accompanying spectral variation during the
$psi_{rm X}$ rotation can be explained by a fluctuation in the physical
conditions, e.g., in the energy distribution of relativistic electrons. The
opposite rotation direction of $psi$ between the X-ray and longer-wavelength
polarization accentuates the conclusion that the X-ray emitting region is
spatially separated from that at longer wavelengths.
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