A multi-frequency study of sub-parsec jets with the Event Horizon Telescope

TL;DR

This study uses multi-frequency VLBI observations from the Event Horizon Telescope to analyze the structure and properties of AGN jets at sub-parsec scales, comparing results with jet models and revealing deviations indicating complex jet physics.

Contribution

It provides the first multi-frequency VLBI analysis of AGN jets at 230 GHz, modeling core properties and testing the Blandford-Königl jet model against high-frequency data.

Findings

Brightness temperature decreases with frequency, deviating from standard models.

Evidence suggests jet acceleration or magnetic energy transfer occurs.

Magnetic field strength varies with distance from the black hole.

Abstract

The 2017 observing campaign of the Event Horizon Telescope (EHT) delivered the first very long baseline interferometry (VLBI) images at the observing frequency of 230 GHz, leading to a number of unique studies on black holes and relativistic jets from active galactic nuclei (AGN). In total, eighteen sources were observed: the main science targets, Sgr A* and M87 along with various calibrators. We investigated the morphology of the sixteen AGN in the EHT 2017 data set, focusing on the properties of the VLBI cores: size, flux density, and brightness temperature. We studied their dependence on the observing frequency in order to compare it with the Blandford-K\"onigl (BK) jet model. We modeled the source structure of seven AGN in the EHT 2017 data set using linearly polarized circular Gaussian components and collected results for the other nine AGN from dedicated EHT publications, complemented by lower frequency data in the 2-86 GHz range. Then, we studied the dependences of the VLBI core flux density, size, and brightness temperature on the frequency measured in the AGN host frame. We compared the observations with the BK jet model and estimated the magnetic field strength dependence on the distance from the central black hole. Our results indicate a deviation from the standard BK model, particularly in the decrease of the brightness temperature with the observing frequency. Either bulk acceleration of the jet material, energy transfer from the magnetic field to the particles, or both are required to explain the observations.