Constraints on Particles and Fields from Full Stokes Observations of AGN

Abstract

Combined polarization imaging of radio jets from Active Galactic Nuclei (AGN) in circular and linear polarization, also known as full Stokes imaging, has the potential to constrain both the magnetic field structure and particle properties of jets. Although only a small fraction of the emission when detected, typically less than a few tenths of a percent but up to as much as a couple of percent in the strongest resolved sources, circular polarization directly probes the magnetic field and particles within the jet itself and is not expected to be modified by external screens. A key to using full Stokes observations to constrain jet properties is obtaining a better understanding of the emission of circular polarization, including its variability and spectrum. We discuss what we have learned so far from parsec scale monitoring observations in the MOJAVE program and from multi-frequency observations of selected AGN.

Relativistic Jets of Blazars

“…Circular polarization is much weaker than linear polarization, typically only some fractions of a percent (e.g., Homan and Lister, 2006) up to a few percent (Homan and Wardle, 2004), which makes it challenging to detect. While some sources show stability of the circular polarization sign over decades (e.g., Homan et al, 2001Homan et al, , 2018, especially at lower (Aller et al, 2003) and higher (Thum et al, 2018) frequencies the variations can be more erratic due to changes in jet opacity or the higher frequency observations probing smaller, possibly more turbulent, length scales of the magnetic fields (Homan et al, 2018). There also seems to be a frequency-dependence on the magnitude of circular polarization, with higher frequency observations often showing higher circular polarization values (Vitrishchak et al, 2008;Thum et al, 2018), which disagrees with expectations for simple homogeneous component models (see Wardle and Homan, 2003, for a review).…”

The content of astrophysical jets

“…The degeneracy is broken by the consideration of circular polarization, which cancels out for a symmetric plasma, but does not of plasma. Observations of circular polarization have been used to infer the presence of pairs in jets, but the interpretation of the results depends on various assumptions if the source is not adequately resolved (Homan et al 2018). Hence, more accurate determinations should wait for detailed polarimetric imaging with the Event Horizon Telescope of M87 (Event Horizon Telescope Collaboration et al 2019).…”

Relativistic Jets of Blazars

“…Circular polarization is much weaker than linear polarization, typically only some fractions of a percent (e.g., Homan and Lister, 2006) up to a few percent (Homan and Wardle, 2004), which makes it challenging to detect. While some sources show stable signs of the circular polarization over decades (e.g., Homan et al, 2001Homan et al, , 2018, especially at lower (Aller et al, 2003) and higher (Thum et al, 2018) frequencies, the variations can be more erratic due to changes in jet opacity or the higher frequency observations probing smaller, possibly more turbulent, length scales of the magnetic fields (Homan et al, 2018). There also seems to be a frequency-dependence of the magnitude of circular polarization, with higher frequency observations often showing higher circular polarization values (Vitrishchak et al, 2008;Thum et al, 2018), which disagrees with expectations for simple homogeneous component models (see Wardle and Homan, 2003, for a review).…”