VLBI and gamma-ray studies of radio galaxies in the TANAMI monitoring program

2018 
Relativistic jets in Active Galactic Nuclei (AGN) are among the most extreme sources of radiation in the universe. They are launched and accelerated by accreting supermassive black holes found in the center of a fraction of galaxies. These jets emit strongly across the whole electromagnetic spectrum, and are especially bright in the radio and gamma-ray bands, where they are among the most studied astrophysical sources. In the simplest models, the production of gamma-rays in radio-loud AGN involves the same relativistic particles that give rise to the radio emission. Therefore, we expect to observe a close connection between the two bands. This has been well-established in studies of large AGN samples. However, such samples are dominated by AGN with well-aligned jets, a sub-class called blazars, where the observed emission is strongly beamed and amplified due to relativistic Doppler effects. While this makes blazars easier to detect, it also poses the challenge of disentangling orientation-dependent effects from the intrinsic physical properties. To this date, there has been no systematic study on the relation between parsec-scale radio emission and gamma-ray properties of the misaligned parent population of blazars, i.e., radio galaxies. In this thesis, I present the first systematic VLBI and gamma-ray monitoring study of a representative sample of radio galaxies with strong compact radio emission, with the aim of exploring the intrinsic relationship between high-energy emission and pc-scale jet properties in AGN. I base the study on the decade(s)-long Very Long Baseline Interferometry (VLBI) monitoring provided by the TANAMI program, the largest multi-epoch observational campaign on radio-loud AGN in the Southern sky. First, I introduce the basic properties of AGN (Chapter 1), and discuss the current understanding of the relation between radio and gamma-ray emission in AGN (Chapter 2). I then introduce the instruments and the corresponding data reduction techniques relevant to this thesis work, i.e., gamma-ray telescopes (Chapter 3) and VLBI arrays (Chapter 4). Chapter 5 presents the results on the evolution of the parsec-scale jet in TANAMI radio galaxies, including milliarcsecond resolution images at 8.4 GHz for several observing epochs, and a jet kinematic analysis to estimate the intrinsic jet speed, viewing angle, and overall evolution, at the highest resolution available for this sample. The information from parsec-scale jet kinematics is combined with gamma-ray flux variability results from the Fermi -LAT space telescope (presented in Chapter 6), and the observed properties in these two bands and their interplay are discussed in Chapter 7. I summarize the main results in Chapter 8.I first discuss individual results on noteworthy TANAMI radio galaxies, such as the FR II radio galaxy Pictor A, the peculiar AGN PKS 0521−36, the TeV source PKS 0625−35, and the first gamma-ray detected young radio galaxy, PKS 1718−649 (Section 7.1). I then combine the TANAMI radio galaxy sample with publicly available results from the MOJAVE survey, the largest VLBI monitoring program of AGN in the northern sky, to study the largest sample to date of radio galaxies with parsec-scale kinematics and gamma-ray information (Section 7.2). Testing for possible correlations between the average radio and gamma-ray properties of radio galaxies, I show that the high-energy emission from the compact jets of radio galaxies is not strongly driven by orientation-dependent Doppler boosting effects, much unlike the situation in their blazar counterparts. However, a significant correlation between gamma-ray flux and radio flux still holds, suggesting a direct physical link between the intrinsic emission properties of AGN jets in the two wavebands.
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