Browsing by Author "Lähteenmäki, Anne, Prof., Aalto University Metsähovi Radio Observatory, Finland"
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- Measurements of magnetic fields in the jets of active galactic nuclei - A tool for testing black hole jet launching models
School of Electrical Engineering | Doctoral dissertation (article-based)(2022) Chamani, WaraActive galactic nuclei (AGNs) are accreting supermassive black holes residing at the centres of galaxies that can launch jets made of magnetised plasma, reaching speeds close to the speed of light. In the context of the Blandford-Znajek mechanism, the magnetic flux paradigm of Sikora and Begelman (2013) can explain the efficient jet production of AGNs via the large-scale poloidal magnetic flux accumulation around a fast-spinning black hole that gives rise to the formation of a magnetically arrested disc (MAD). In the above scenario, magnetic fields play an important role in launching relativistic jets in AGNs and possibly producing variability in their radio emission. This thesis aims to test the magnetic flux paradigm observationally by using the so-called core-shift method to estimate the parsec-scale jet magnetic fields close to the black hole. Furthermore, it addresses how to reliably infer magnetic field strengths by measuring core-shift (rcore ∝ ν−1/kr) variability. The magnetic flux paradigm has been effectively tested with a sample of radio-loud AGNs, not including radio-intermediate and radio-quiet AGNs. In this thesis, the model was tested for the first time with a radio-intermediate quasar III Zw 2. For this, the study utilised (single epoch) multi-frequency Very Long Baseline Array (VLBA) observations to infer the magnetic flux and joint XMM-Newton and NuSTAR X-ray observations to measure the black hole spin by fitting a relativistic reflection model to the reflection spectrum. The results on III Zw 2 revealed that the magnetic flux value predicted by the MAD model is higher by a factor of five compared to the upper limit of the magnetic flux. Thus, the source did not reach the MAD state despite harbouring a fast-spinning black hole. These findings show that the magnetic flux controls the jet production efficiency, as predicted by the magnetic flux paradigm. Our study on III Zw 2 presents a new observational method for testing the magnetic flux paradigm and proposes extending such a study to radio-quiet AGNs. The investigation of the core-shift time variability was performed on the blazar 3C454.3. For this purpose, multi-frequency and multi-epoch (19 epochs) VLBA observations were utilised. Additionally, radio, near-infrared and optical observations on 3C454.3 were employed to study variability and waveband correlations. Our results on 3C454.3 show significant time variability of the core-shift magnitude, confirming a previous study. We have demonstrated for the first time the variability of the core-shift index kr also finding kr ≤ 1 during flaring and quiescent states in the study period. These results indicate deviations from the ideal conditions of equipartition and conical jet shape (kr = 1) of the Blandford and K ̈onigl jet model. Hence, both conditions might not hold simultaneously. Our study suggests that the derived magnetic field parameters are reliable only as long as the core-shift observations confirm kr = 1. - Narrow-line Seyfert 1 galaxies - Observational and statistical analysis
School of Electrical Engineering | Doctoral dissertation (article-based)(2018) Järvelä, EmiliaNarrow-line Seyfert 1 (NLS1) galaxies are young active galactic nuclei (AGN). They harbour low-mass black holes accreting close to the Eddington limit and are preferentially hosted by spiral galaxies. So far ~20 NLS1 galaxies have been detected at gamma-rays, confirming the presence of powerful relativistic jets in them. This contradicts the conventional view that only supermassive black holes residing in massive ellipticals are able to launch relatistic jets, and therefore a revision of the evolution and unification schemes of AGN is required. In this thesis large samples of NLS1 galaxies are examined, complemented by targeted studies of smaller samples. Novel radio, near-infrared, and large-scale environment data were obtained and, together with archival multifrequency data, used for extensive statistical studies such as correlation analyses. Principal component analysis (PCA) emerged as an excellent tool for studying the diverse NLS1 samples. We monitored large samples of NLS1 galaxies at 37 GHz in Metsähovi Radio Observatory. This is the largest observing programme of NLS1 galaxies at radio frequencies. 19% of sources selected based on their radio properties and 12% of sources selected based on other criteria were detected, including sources previously (mis)classified as radio-silent. As detections at 37~GHz are indicators of radio emission from a jet, this implies that powerful jets in NLS1 galaxies are more frequent than previously assumed. We also found that the jets of flat-spectrum NLS1 galaxies are less powerful than those of blazars, but when scaled by the black hole mass the jet powers become comparable. Additionally, we discovered a new gamma-ray emitting source in the radio-silent NLS1 sample. We show that the large-scale environment density affects the radio properties of NLS1 galaxies and that jetted NLS1 sources are more frequently found in denser surroundings. However, on average jetted NLS1 galaxies reside in significantly less dense large-scale environments than other jetted AGN, proving that powerful jets can be triggered in diverse environments. In addition a parameter describing the large-scale environment was included in the PCA of AGN for the first time. Our near-infrared imaging of sources detected at 37 GHz almost tripled the number of jetted NLS1 galaxies with known host morphologies. All observed NLS1 nuclei reside in spiral galaxies. The fraction of mergers in this sample is significantly higher than among non-jetted NLS1 galaxies, suggesting that interaction may play a role in triggering the jet. The heterogeneity of the NLS1 population could be explained by disparate evolutionary stages induced by interaction.