Synchrotron radiation of self-collimating relativistic MHD jets

TL;DR

This paper models synchrotron radiation from relativistic MHD jets using simulations, deriving observable signatures like polarization and Faraday rotation, and explores how these can reveal jet and black hole properties.

Contribution

It presents detailed relativistic MHD simulations of collimating jets with helical magnetic fields and computes their synchrotron emission signatures for the first time.

Findings

Non-monotonic RM profiles detectable at 100 Schwarzschild radii resolution.

Relativistic aberration causes polarization asymmetries that can indicate black hole spin.

Jet polarization structure can be understood beyond the previously predicted bi-modality.

Abstract

The goal of this paper is to derive signatures of synchrotron radiation from state-of-the-art simulation models of collimating relativistic magnetohydrodynamic (MHD) jets featuring a large-scale helical magnetic field. We perform axisymmetric special relativistic MHD simulations of the jet acceleration region using the PLUTO code. The computational domain extends from the slow magnetosonic launching surface of the disk up to 6000^2 Schwarzschild radii allowing to reach highly relativistic Lorentz factors. The Poynting dominated disk wind develops into a jet with Lorentz factors of 8 and is collimated to 1 degree. In addition to the disk jet, we evolve a thermally driven spine jet, emanating from a hypothetical black hole corona. Solving the linearly polarized synchrotron radiation transport within the jet, we derive VLBI radio and (sub-) mm diagnostics such as core shift, polarization structure, intensity maps, spectra and Faraday rotation measure (RM), directly from the Stokes parameters. We also investigate depolarization and the detectability of a lambda^2-law RM depending on beam resolution and observing frequency. We find non-monotonic intrinsic RM profiles which could be detected at a resolution of 100 Schwarzschild radii. In our collimating jet geometry, the strict bi-modality in polarization direction (as predicted by Pariev et al.) can be circumvented. Due to relativistic aberration, asymmetries in the polarization vectors across the jet can hint to the spin direction of the central engine.