Polarized blazar X-rays imply particle acceleration in shocks

Abstract

Most of the light from blazars, active galactic nuclei with jets of magnetized plasma that point nearly along the line of sight, is produced by high-energy particles, up to around 1 TeV. Although the jets are known to be ultimately powered by a supermassive black hole, how the particles are accelerated to such high energies has been an unanswered question. The process must be related to the magnetic field, which can be probed by observations of the polarization of light from the jets. Measurements of the radio to optical polarization—the only range available until now—probe extended regions of the jet containing particles that left the acceleration site days to years earlier1–3, and hence do not directly explore the acceleration mechanism, as could X-ray measurements. Here we report the detection of X-ray polarization from the blazar Markarian 501 (Mrk 501). We measure an X-ray linear polarization degree ΠX of around 10%, which is a factor of around 2 higher than the value at optical wavelengths, with a polarization angle parallel to the radio jet. This points to a shock front as the source of particle acceleration and also implies that the plasma becomes increasingly turbulent with distance from the shock.

Relativistic Collisionless Shocks in Inhomogeneous Magnetized Plasmas

“…Finally, an important feature of corrugated shocks that our simulations reveal is the formation of coherent, large-scale magnetized structures in the downstream region. This is particularly important in light of the high X-ray polarization found in blazar Mrk 501 (Liodakis et al 2022), which requires ordered magnetic field at the site of particle acceleration. These results appeal to reconsider relativistic shocks as plausible sites for particle acceleration in magnetized astrophysical environments, such as relativistic jets.…”

Shock Corrugation to the Rescue of the Internal Shock Model in Microquasars: The Single-scale Magnetohydrodynamic View

“…We caution that these specific values would only apply to the case of a single-shell collision. Once a lot of events overlap (with some collisions likely being off-center, at different inclinations to the jet axis), the expected total polarization fraction of the jet emission should be lower, and could be comparable to the X-ray polarization recently detected by the Imaging X-ray Polarimetry Explorer (IXPE; Liodakis et al 2022). Nonetheless, unless another source of polarized radiation contributes to the observed bands, the X-ray emission should remain more strongly polarized than the optical band.…”

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.…”