Kavli Affiliate: Bradford A. Benson
| First 5 Authors: Jan Röder, Maciek Wielgus, Andrei P. Lobanov, Thomas P. Krichbaum, Dhanya G. Nair
| Summary:
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.
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