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The title of the thesis: Long-term radio monitoring of active galactic nuclei: Characteristic timescales and improved analysis methods

Thesis defender: Sofia Kankkunen
Opponent: Dr. Phil Uttley, Anton Pannekoek Institute for Astronomy, University of Amsterdam, The Netherlands
Custos: Prof. Anne Lähteenmäki, Aalto University School of Electrical Engineering

Active galactic nuclei (AGN) are extremely luminous centres of active galaxies, and they are powered by a supermassive black hole. The luminosity of AGN is highly time-variable and studying this variability across different wavelengths allows understanding their structures, all the way from the accretion disk to the massive plasma jets shooting from the black holes. In the radio domain, the source of the observed emission and its variations are these jets where the plasma travels near the speed of light.

In this doctoral thesis, the long-term radio variability of AGN has been studied using over 40 years of data observed at the Aalto University Metsähovi Radio Observatory. The timeseries used in this thesis are uniquely long, providing an unprecedented possibility to study the extremely slow variability present in radio emission. The characterisation of variability was also analysed from a methodological standpoint, leading to a broader understanding on the methodology and how the results are connected.

In the thesis, the variability of the radio emission was characterised more accurately than has been possible before. The results will help in understanding both the variability mechanisms as well as the differences between source types. The observed variability was also analysed in connection to the physical parameters of the central engine. A preliminary association was found linking the black hole mass and the rate with which it accretes matter to the jet emission.

The results from this doctoral thesis provide an improved understanding on the long-term variability of AGN in the radio domain. The found connections between radio variability and the central engine supply new evidence on how the central engine modulates the brightness variations observed in the jet, providing a basis for follow-up studies. The need for continuous long-term monitoring is also highlighted as even over 40 years of observations are not enough for characterising the long-term variability of many of the sources.

Key words: active galactic nuclei, quasar, radio astronomy, relativistic jet, supermassive black hole

Thesis available for public display 7 days prior to the defence at .