Signatures of large-scale magnetic fields in AGN jets: transverse asymmetries

TL;DR

This paper explores how large-scale helical magnetic fields in AGN jets create observable asymmetries in polarization, intensity, and spectral profiles, which can reveal the magnetic field structure and explain features seen in sources like M87.

Contribution

It demonstrates how transverse asymmetries in AGN jet emissions can be used to infer the presence and properties of large-scale helical magnetic fields, supported by both simulations and observations.

Findings

Asymmetries correlate with magnetic field handedness and viewing angle.

Edge-brightened profiles similar to M87 are explained by the model.

Observed asymmetries in 3C 78 and NRAO 140 support the theory.

Abstract

We investigate the emission properties that a large-scale helical magnetic field imprints on AGN jet synchrotron radiation. A cylindrically symmetric relativistic jet and large-scale helical magnetic field produce significant asymmetrical features in transverse profiles of fractional linear polarization, intensity, Faraday rotation, and spectral index. The asymmetrical features of these transverse profiles correlate with one another in ways specified by the handedness of the helical field, the jet viewing angle (theta_ob), and the bulk Lorentz factor of the flow (Gamma). Thus, these correlations may be used to determine the structure of the magnetic field in the jet. In the case of radio galaxies (theta_ob~1 radian) and a subclass of blazars with particularly small viewing angles (theta_ob << 1/Gamma), we find an edge-brightened intensity profile that is similar to that observed in the radio galaxy M87. We present observations of the AGNs 3C 78 and NRAO 140 that display the type of transverse asymmetries that may be produced by large-scale helical magnetic fields.