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Abstract How astrophysical systems translate the kinetic energy of bulk motion into the acceleration of particles to very high energies is a pressing question. SS 433 is a microquasar that emits TeVγ-rays indicating the presence of high-energy particles. A region of hard X-ray emission in the eastern lobe of SS 433 was recently identified as an acceleration site. We observed this region with the Imaging X-ray Polarimetry Explorer and measured a polarization degree in the range 38%–77%. The high polarization degree indicates the magnetic field has a well-ordered component if the X-rays are due to synchrotron emission. The polarization angle is in the range −12° to +10° (east of north), which indicates that the magnetic field is parallel to the jet. Magnetic fields parallel to the bulk flow have also been found in supernova remnants and the jets of powerful radio galaxies. This may be caused by interaction of the flow with the ambient medium. 

Aims.We aim to probe the magnetic field geometry and particle acceleration mechanism in the relativistic jets of supermassive black holes. Methods.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 from 2022 December 6–8. During the IXPE observation, we also monitored Mrk 421 usingSwift-XRT and obtained a single observation withXMM-Newtonto improve the X-ray spectral analysis. The time-averaged X-ray polarization was determined consistently using the event-by-event Stokes parameter analysis, spectropolarimetric fit, and maximum likelihood methods. We examined the polarization variability over both time and energy, the former via analysis of IXPE data obtained over a time span of 7 months. Results.We detected X-ray polarization of Mrk 421 with a degree of Π_X = 14 ± 1% and an electric-vector position angleψ_X = 107 ± 3° in the 2–8 keV band. From the time variability analysis, we find a significant episodic variation inψ_X. During the 7 months from the first IXPE pointing of Mrk 421 in 2022 May,ψ_Xvaried in the range 0° to 180°, while Π_Xremained relatively constant within ∼10–15%. Furthermore, a swing inψ_Xin 2022 June was accompanied by simultaneous spectral variations. The results of the multiwavelength polarimetry show that Π_Xwas generally ∼2–3 times greater than Π at longer wavelengths, whileψfluctuated. Additionally, based on radio, infrared, and optical polarimetry, we find that the rotation ofψoccurred in the opposite direction with respect to the rotation ofψ_Xand over longer timescales at similar epochs. Conclusions.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. We considered two versions of the model, one with linear and the other with radial stratification geometry, to explain the rotation ofψ_X. The accompanying spectral variation during theψ_Xrotation can be explained by a fluctuation in the physical conditions, for example in the energy distribution of relativistic electrons. The opposite rotation direction ofψbetween the X-ray and longer wavelength polarization accentuates the conclusion that the X-ray emitting region is spatially separated from that at longer wavelengths. Moreover, we identify a highly polarized knot of radio emission moving down the parsec-scale jet during the episode ofψ_Xrotation, although it is unclear whether there is any connection between the two events. 

Abstract Observations of linear polarization in the 2–8 keV energy range with the Imaging X-ray Polarimetry Explorer (IXPE) explore the magnetic field geometry and dynamics of the regions generating nonthermal radiation in relativistic jets of blazars. These jets, particularly in blazars whose spectral energy distribution peaks at X-ray energies, emit X-rays via synchrotron radiation from high-energy particles within the jet. IXPE observations of the X-ray-selected BL Lac–type blazar 1ES 1959+650 on 2022 May 3–4 showed a significant linear polarization degree of Π_x= 8.0% ± 2.3% at an electric-vector position angleψ_x= 123° ± 8°. However, on 2022 June 9–12, only an upper limit of Π_x≤ 5.1% could be derived (at the 99% confidence level). The degree of optical polarization at that time, Π_O∼ 5%, is comparable to the X-ray measurement. We investigate possible scenarios for these findings, including temporal and geometrical depolarization effects. Unlike some other X-ray-selected BL Lac objects, there is no significant chromatic dependence of the measured polarization in 1ES 1959+650, and its low X-ray polarization may be attributed to turbulence in the jet flow with dynamical timescales shorter than 1 day.