Browsing by Author "Lähteenmäki, Anne, Prof., Aalto University, Metsähovi Radio Observatory, Finland"
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- Multifrequency connection in blazars: Tool for studying the location and emission mechanisms
School of Electrical Engineering | Doctoral dissertation (article-based)(2016) Ramakrishnan, VenkatesshBlazars are a sub-class of AGN with relativistic jets oriented towards the line of sight of the observer. The radiation mechanism in the radio, and sometimes in optical and X-rays, are dominated by synchrotron processes with high-energy emission arising from the inverse-Compton scattering of photons by the relativistic electrons or through hadronic processes. The latter scenario is one of the topics debated in the blazar research, primarily owing to the location of the seed photons for the high-energy emission. This problem is contemplated in this thesis for a sample of blazars. A proper understanding of the physical mechanisms responsible for variability is contingent upon a mathematical and statistical description of the phenomena. Hence, the variability of all light curves was investigated using power spectral density and Bayesian blocks, and its connection using the discrete correlation function. The connection between the variability at 37 GHz and gamma-ray light curves was studied in 55 blazars. The average time delay from this study showed that the variations in the radio lagged behind those at gamma rays. However, by taking the size of the radio core into account the gamma-ray emission region was localised to the radio core in the majority of sources. In another work, the cross-correlations were performed for 15 blazars using light curves at 37 GHz, 95 GHz, R-band and gamma rays. Due to the shorter time span of the light curves and undersampled optical data in some cases, significant correlations were obtained only in a few sources. The variations at 37 GHz and 95 GHz were simultaneous in most of the sources, indicating that the opacity constraints at both these frequencies were similar. The connection between the events in the radio through gamma rays was studied in the blazars 1156+295, Mrk 421 and OJ 248. From the correspondence of the activity at both radio and gamma rays in 1156+295, and also based on the ejection time of the shocks from the Very Long Baseline Array analysis, a causal connection constraining the gamma-ray flares to the parsec-scale radio jet is established. The physical explanation for the radio variability in Mrk 421 is discussed by modelling the radio and gamma-ray light curve under a one-zone SSC framework. For OJ 248, a multifrequency connection for the 2012 gamma-ray flare is explored using cross-correlations. To understand the synchrotron processes, the radio spectra of 104 AGN from Planck satellite observations are analysed. The shape of most of the spectra is consistent with those predicted by the shock-in-jet model. The overall conclusion from this thesis is that in most of the blazars studied the gamma-ray emission arises from the parsec-scale radio jet, which supports the SSC mechanism (through one- or multi-zone models) for the high-energy emission. - 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.