Determining the composition of radio plasma via circular polarization: the prospects of the Cygnus A hot spots

Abstract

The composition of the relativistic plasma produced in active galactic nuclei and ejected via powerful jets into the interstellar/intergalactic medium is still a major unsettled issue. It might be a positron-electron plasma in case the plasma was created by pair production in the intense photon fields near accreting super-massive black holes. Alternatively, it might be an electron-proton plasma in case magnetic fields lift and accelerate the thermal gas of accretion discs into relativistic jets as the recent detection of $\gamma$-rays from blazars indicates. Despite various attempts to unambiguously establish the composition of the relativistic jets, this remains a major unknown. Here, we propose a way to settle the question via sensitive measurements of circular polarization (CP) in the radio emission of the hot spots of bright radio galaxies like Cygnus A. The CP of synchrotron emission is determined by the circular motions of the radiating relativistic leptons. In case of charge symmetric energy spectra of a electron-positron plasma, it should be exactly zero. In case of an electron-proton plasma the electrons imprint their gyration onto the CP and we expect the hot spots of Cygnus A to exhibit a fractional CP at a level of $10^{-3}\,(ν/\mbox{GHz})^{-{1}/{2}}$, which is challenging to measure, but not completely unfeasible.

6 pages, no figures, accepted by JCAP, erratum with respect to accepted version applied, numerical corrections leading to a stronger effect

References in corpus (14)

• The blazar TXS 0506+056 associated with a high-energy neutrino: insights into extragalactic jets and cosmic ray acceleration
• Particle content, radio-galaxy morphology and jet power: all radio-loud AGN are not equal
• Shedding New Light on the 3C 273 Jet with the Spitzer Space Telescope
• A representative survey of the dynamics and energetics of FRII radio galaxies
• LOFAR imaging of Cygnus A -- Direct detection of a turnover in the hotspot radio spectra
• Radio Circular Polarization Produced in Helical Magnetic Fields in Eight Active Galactic Nuclei
• Ultra-High-Energy Cosmic Rays from Radio Galaxies
• Fermi non-detections of four X-ray jet sources and implications for the IC/CMB mechanism
• The Particle Content of Extragalactic Jets
• The TeV emission of Ap Librae: a hadronic interpretation and prospects for CTA
• Strong constraint on hadronic models of blazar activity from Fermi and IceCube stacking analysis
• Modelling blazar flaring using a time-dependent fluid jet emission model - an explanation for orphan flares and radio lags
• The Galaxy in circular polarization: all-sky radio prediction, detection strategy, and the charge of the leptonic cosmic rays
• A Photo-Hadronic Model of the Large Scale Jet of PKS 0637-752

Cited by in corpus (3)

• Black Hole Magnetic Fields and Their Imprint on Circular Polarization Images
• Determining the Composition of Relativistic Jets from Polarization Maps
• From electrons to Janskys: Full stokes polarized radiative transfer in 3D relativistic particle-in-cell jet simulations