Georgios F. Paraschos

PhD Project: High frequency observations of AGN

PhD Supervisor: Dr. Thomas P. Krichbaum

Main Collaborators: T. P. Krichbaum, J.-Y. Kim, J. Oh, J. A. Hodgson, E. Ros, G. Witzel, J. A. Zensus

Georgios F. Paraschos in 2021

Project description
The main focus of my PhD project is the study of the structure and structural variability οf nearby γ-ray active AGN jets using VLBI imaging at the highest possible resolution, in order to better understand the details of jet launching and acceleration, in relation to the observed broad band AGN activity throughout the electromagnetic spectrum. Of particular interest is to test jet launching models, understand the origin of the high-energy (γ-ray) activity (observed by the FERMI satellite) and to search for correlations between flaring activity and jet propagation on sub-pc scales.

VLBI imaging at the shortest possible wavelengths (7 mm, 3.4 mm) offers a more direct view into the – at longer wavelength synchrotron self-absorbed – AGN core regions, with an angular resolution of up to 50 μas. It is therefore possible to study and image spatial scales of much less than a parsec in size. With mm-VLBI the direct imaging of the vicinity of SMBHs thus becomes possible. Depending on black hole mass and distance, the angular resolution provided by mm-VLBI translates into spatial scales of several ten to a few thousand Schwarzschild radii. These scales currently cannot be directly imaged by any other method other than mm-VLBI. Current AGN jet-models assume that on these scales the ultra-relativistic radio jets are formed and that the initial jet acceleration happens through electromagnetic forces. The mm-VLBI imaging of the jets therefore can be used to better understand the physical details of jet formation, initial acceleration and downstream jet expansion and propagation. In particular it can help to find out if and where the bulk energy is converted from magnetic to kinetic. The combination of the observed spectral turnover frequency with flux and size measurements from VLBI can yield a measure of the kinetic and magnetic energy balance. Knowing this will help to constrain and test existing theoretical jet launching scenarios.

Through the combination of VLBI data obtained at 7 mm and 3mm with archival data at longer cm-wavelength (e.g. 2 cm), it is planned to perform a spectral decomposition of the inner jet region. This will help to discriminate between the synchrotron self-absorbed jet base, its possible stratification and the occurrence of moving or stationary shocks at the jet base. The latter are expected to form through jet re-collimation in an over-pressured jet nozzle, and are suspected to be at least partially responsible for γ-ray production.

Two are the main research objectives of this project. Firstly the analysis of the high angular resolution 15, 22, 43 and 86 GHz VLBI data of 3C84 obtained from available observations with the aim to determine the structure of the inner jet region (< 3 mas), its variability and kinematics. A particular focus will be the study of the sub-mas scale structure of the VLBI core of 3C84, which is elongated almost perpendicular to the overall jet direction and whose orientation changes with time. The GMVA imaging should shed further light on the question if and how the inner jet connects to the VLBI core, and whether the size of the latter is a useful quantity for testing launching models. And secondly, the study of the properties of the jet nozzle and inner jet region of some more distant and gamma-ray loud AGN (eg. 3C120, 3C345, 3C454.3), for which good GMVA data are available. Besides the correlation of the jet kinematics with the known broad-band (radio to gamma-ray) flux density variability, a focus will be on the determination of the polarisation properties and the spatial jet curvature in order to clarify if the latter is due to jet precession or propagation of plasma instabilities.

About me
I was born in Athens, Greece. I have always been interested in natural sciences, thus studying physics was a sequential choice for me. My bachelor and master theses revolved around the imaging, analysis and modelling of molecular emission lines in a sample of galaxies. The main fitting codes I worked with are called RADEX and 3D-PDR, which I combined with self-written python scripts. My interest in radioastronomy and VLBI were triggered, when I attended the University of Bonn for a semester as an Erasmus+ exchange student. There I decided to pursue a PhD utilising the VLBI technique. I started my PhD at MPIfR in October of 2019.

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