Limb-Brightened Jet in M87 from Anisotropic Nonthermal Electrons

TL;DR

This paper introduces a new model for relativistic jets that explains limb-brightening in M87 by considering anisotropic nonthermal electrons aligned with magnetic fields, validated through simulations and radiative transfer.

Contribution

It proposes an anisotropic electron distribution model that reproduces limb-brightening in M87 jets, integrating it into GRMHD and GRFFE simulations for realistic jet imaging.

Findings

Limb-brightening arises from electron velocities aligned with magnetic fields.

The model predicts jet images at various scales and frequencies.

Predictions for horizon-scale images at 230 and 345 GHz are provided.

Abstract

Very long baseline interferometry observations reveal that relativistic jets like the one in M87 have a limb-brightened, double-edged structure. Analytic and numerical models struggle to reproduce this limb-brightening. We propose a model in which we invoke anisotropy in the distribution function of synchrotron-emitting nonthermal electrons such that electron velocities are preferentially directed parallel to magnetic field lines, as suggested by recent particle-in-cell simulations of electron acceleration and the effects of synchrotron cooling. We assume that the energy injected into nonthermal electrons is proportional to the jet Poynting flux, and we account for synchrotron cooling via a broken power-law energy distribution. We implement our emission model in both general relativistic magnetohydrodynamic (GRMHD) simulations and axisymmetric force-free electrodynamic (GRFFE) jet models and produce simulated jet images at multiple scales and frequencies using polarized general relativistic radiative transfer. We find that the synchrotron emission is concentrated parallel to the local helical magnetic field and that this feature produces limb-brightened jet images on scales ranging from tens of microarcseconds to hundreds of milliarcseconds in M87. We present theoretical predictions for horizon-scale M87 jet images at 230 and 345 GHz that can be tested with next generation instruments. Due to the scale-invariance of the GRMHD and GRFFE models, our emission prescription can be applied to other targets and serve as a foundation for a unified description of limb-brightened synchrotron images of extragalactic jets.