Observable Emission Features of Black Hole GRMHD Jets on Event Horizon Scales

TL;DR

This paper models the emission features of black hole jets on event horizon scales using GRMHD, predicting a luminous stagnation surface where energetic electrons are injected, which can be tested by future VLBI observations.

Contribution

It introduces a conservative electron transport formulation in GRMHD jets, predicting observable emission features and the significance of the stagnation surface for future observations.

Findings

The emission region is finite and elongated along the jet axis.

Regions near the stagnation surface are the most luminous.

Future VLBI observations can verify the model's predictions.

Abstract

The general-relativistic magnetohydrodynamical (GRMHD) formulation for black hole-powered jets naturally gives rise to a stagnation surface, wherefrom inflows and outflows along magnetic field lines that thread the black hole event horizon originate. We derive a conservative formulation for the transport of energetic electrons which are initially injected at the stagnation surface and subsequently transported along flow streamlines. With this formulation the energy spectra evolution of the electrons along the flow in the presence of radiative and adiabatic cooling is determined. For flows regulated by synchrotron radiative losses and adiabatic cooling, the effective radio emission region is found to be finite, and geometrically it is more extended along the jet central axis. Moreover, the emission from regions adjacent to the stagnation surface is expected to be the most luminous as this is where the freshly injected energetic electrons concentrate. An observable stagnation surface is thus a strong prediction of the GRMHD jet model with the prescribed non-thermal electron injection. Future millimeter/sub-millimeter (mm/sub-mm) very-long-baseline interferometric (VLBI) observations of supermassive black hole candidates, such as the one at the center of M87, can verify this GRMHD jet model and its associated non-thermal electron injection mechanism.