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 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.

X-Ray Polarization of the BL Lacertae Type Blazar 1ES 0229+200

“…Blazar emission is usually characterized by two broad spectral humps from radio to X-rays and X-rays to TeV γ-rays. The low-energy hump is interpreted as synchrotron radiation from energetic electrons, which for the most extreme blazars can peak at energies of ∼1-50 keV (Costamante et al 2001;Di Gesu et al 2022;Liodakis et al 2022).…”

“…It is particularly important for the study of extragalactic jets, as it can directly test particle acceleration and emission mechanisms in blazars by providing information on the geometry of the magnetic fields involved (Zhang & Böttcher 2013;Zhang et al 2016;Liodakis et al 2019;Peirson et al 2022). During the first year (2022) of IXPE, several observations of Mrk 501 (Liodakis et al 2022) and Mrk 421 (Di Gesu et al 2022) were obtained, with more HSPs scheduled to be observed in the second year. Here we present the first X-ray polarization observations of 1ES 0229+200.…”

Linear Polarization Signatures of Particle Acceleration in High-Synchrotron-Peak Blazars

“…Adopting, as before, δ = 20, B = 0.02 G, and an SED peak ratio = 1, we estimate t cross ∼ 0.6 days for the 4 keV X-ray-emitting region and ~28 days at the optical R band. The former implies that IXPE should have detected variations in the X-ray polarization in both Mkn421 and Mkn501 during the first observations in 2022, contrary to the observations [30,31]. The expected optical variability time scale of ~4 weeks can be compared with the changes in optical polarization when the object was observed with adequate time coverage.…”

“…As discussed in §2 above, this orientation can occur in a few different ways: a moving or standing shock, which partially Recently, X-ray linear polarization measurements have been made possible by the Imaging X-ray Polarimetry Explorer (IXPE) mission [29]. IXPE detected polarization of both Mkn421 [30] and Mkn501 [31] over the energy range of 2-8 keV in 2022. In the case of Mkn501, P x = 10 ± 2% and χ x = 134 • ± 5 • during one 2-day measurement and P x = 11 ± 2% and χ x = 115 • ± 4 • during another 2-day pointing 18 days later.…”

X-Ray Polarization Observations of BL Lacertae

“…Starting in 2022 January, the Imaging X-ray Polarimetry Explorer (IXPE; Weisskopf et al 2010Weisskopf et al , 2016Weisskopf et al , 2022 has been carrying out such measurements. Detection by IXPE of high-synchrotron-peak sources like Mrk 501 and Mrk 421 (Di Gesu et al 2022;Liodakis et al 2022b) has revealed stronger polarization at X-rays than at longer wavelengths. This is consistent with emission by high-energy electrons that are accelerated at a shock front with partially ordered magnetic fields, after which they are advected to regions with more turbulent fields.…”