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

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

Particle acceleration mechanisms in supermassive black hole jets, such as shock acceleration, magnetic reconnection, and turbulence, are expected to have observable signatures in the multiwavelength 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-synchrotron-peaked BL Lac objects (HSPs). We report the discovery of X-ray linear polarization (degree Πx = 15% ± 2% and electric vector position angle ψ x = 35° ± 4°) 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 Πx compared to longer wavelengths, and the absence of significant polarization variability, suggest a shock is 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.

超大质量黑洞喷流中的粒子加速机制，比如激波加速、磁重联和湍流，预计会在耀变体的多波段偏振特性中留下可观测的特征。近期发射的X射线成像偏振探测器（IXPE）使我们首次能够利用X射线波段（2 - 8 keV）的偏振来探测高同步辐射峰值的蝎虎天体（HSPs）中喷流同步辐射的特性。我们报告了在平均X射线通量状态下，从HSP Mrk 421的喷流中发现了X射线线偏振（偏振度Πx = 15% ± 2%，电矢量方位角ψx = 35° ± 4°）。同时，发现光学、红外和毫米波段的偏振度至少低3倍。在IXPE观测期间，该源的X射线通量增加了2.2倍，而偏振行为显示无变化。与较长波长相比，Πx的值更高，且偏振无明显变化，这表明在观测期间，Mrk 421喷流中最有可能的X射线发射位置是激波。多波段偏振特性与能量分层的电子群一致，其中在较长波长发射的粒子距离加速位置更远，它们所处的磁场更无序。