VLBI and total flux density investigations of the structure of active galactic nuclei

Abstract

This doctoral thesis studies the structure of active galactic nuclei (AGN) using very long baseline interferometry (VLBI) at high radio frequencies (22 - 86 GHz). The VLBI observations are complemented using information from total flux density (TFD) observations at the corresponding frequencies and the TFD variations are compared to the structure changes seen in the VLBI images.

The basis of this work is the data from the three epoch VLBI monitoring project, which produced 45 images and parameters of 15 extragalactic sources at 22 GHz.

The maximum intrinsic brightness temperature for synchrotron sources is assumed to be limited by the inverse Compton catastrophe to ≅ 1012 K. We have presented two new methods to estimate this limit using total flux density observations, synchrotron-Self-Compton X-ray fluxes and shock parameters from VLBI observations. Using several data sets, we find that both methods yield a value of ≤ 1011 K, which is consistent with the equipartition limit of 1010 - 1011 K.

We have developed a new method for estimating the geometry of the Universe using the linear sizes of shocks in AGN jets. The angular sizes of the shocks from VLBI observations are normalized using intrinsic diameter estimates from TFD observations. Using a very small set of test data, we verify that the accuracy of the derived values compare favourably with traditional methods using much larger samples.

A clear connection between radio and gamma-ray flares and VLBI component ejections was found. Furthermore, it was shown that the origin of the gamma-ray flares is in the shocks and that they can not be produced close to the core of the AGN.

A helical structure was found in the jet of CTA 102, a High Polarised Quasar (HPQ). The apparent proper motions in the jet varied from 4.2 to 13.8 times the speed of the light. This work combined the work of several years of observation, covering wavelengths from 1.3 to 12 mm and angular resolutions from 50 to 500 microarcseconds.

The first 2mm VLBI experiment between Pico Veleta and Metsähovi was performed and correlation fringes were detected on a baseline of 1.55 billion wavelengths. The success of this experiment shows that VLBI observations are possible at very high frequencies leading to very high resolution and enabling to produce images of the fine structure of the cores of the AGN.