Radio variability of active galactic nuclei : analysis of long-term multifrequency data

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Doctoral thesis (article-based)
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TKK radio science and engineering publications, 6
A large sample of Active Galactic Nuclei (AGN) has been monitored on a regular basis in Metsähovi Radio Observatory since the 1980s. These observations form an extensive database of about 100 brightest Northern compact extragalactic objects at high radio frequencies. In this thesis the Metsähovi observations are complemented with lower frequency radio data from the University of Michigan Radio Astronomy Observatory, and with higher frequency data from the literature. The main focus in this thesis is to understand the long-term radio variability behaviour of AGN by using various methods. Several statistical methods, and their capability to detect variability timescales, are examined in detail. In addition, the flux curves are examined visually to study the flare characteristics, and to verify the timescales obtained. The results show that variability behaviour of AGN is complex, and multiple characteristic timescales are common. Flares in the radio regime are long-lasting, on average, 2.5 years at 37 GHz. In addition, large outbursts occur quite rarely, only once in every four to six years. None of the sources were seen to exhibit strict periodicities. These results together show that long-term monitoring is essential in understanding the true behaviour of AGN. In this thesis, differences between various statistical timeseries analysis methods are investigated in detail for the first time using a large sample of sources and long datasets. Especially the wavelet method has never before been used to study a large sample of AGN at high radio frequencies. A useful property of wavelets is that they show when the timescale has been present and how it has changed. Thus, it is ideal for studying sources which are not strictly periodic and which change their behaviour. The long-term total flux density observations are also used to calculate the Doppler boosting factors, Lorentz factors and viewing angles of the jets. Quasars are found to be more Doppler boosted and to have faster jets than BL Lacertae objects. The various parameters obtained in this thesis, describing the characteristics of AGN, can be further used to study the physical processes and the relationships between different emission regions in these sources.
radio astronomy, active galactic nuclei, statistical methods
Other note
  • [Publication 1]: T. Hovatta, M. Tornikoski, M. Lainela, H. J. Lehto, E. Valtaoja, I. Torniainen, M. F. Aller, and H. D. Aller. 2007. Statistical analyses of long-term variability of AGN at high radio frequencies. Astronomy & Astrophysics, volume 469, number 3, pages 899-912. © 2007 European Southern Observatory (ESO). By permission.
  • [Publication 2]: T. Hovatta, E. Nieppola, M. Tornikoski, E. Valtaoja, M. F. Aller, and H. D. Aller. 2008. Long-term radio variability of AGN: flare characteristics. Astronomy & Astrophysics, volume 485, number 1, pages 51-61. © 2008 European Southern Observatory (ESO). By permission.
  • [Publication 3]: T. Hovatta, H. J. Lehto, and M. Tornikoski. 2008. Wavelet analysis of a large sample of AGN at high radio frequencies. Astronomy & Astrophysics, volume 488, number 3, pages 897-903. © 2008 European Southern Observatory (ESO). By permission.
  • [Publication 4]: T. Hovatta, E. Valtaoja, M. Tornikoski, and A. Lähteenmäki. 2009. Doppler factors, Lorentz factors and viewing angles for quasars, BL Lacertae objects and radio galaxies. Astronomy & Astrophysics, volume 494, number 2, pages 527-537. © 2009 European Southern Observatory (ESO). By permission.
  • [Publication 5]: Elina Nieppola, Talvikki Hovatta, Merja Tornikoski, Esko Valtaoja, Margo F. Aller, and Hugh D. Aller. 2009. Long-term variability of radio-bright BL Lacertae objects. The Astronomical Journal, volume 137, number 6, pages 5022-5036. © 2009 by authors and © 2009 American Astronomical Society (AAS). By permission.
  • [Errata file]: Errata of publication 4