Abstracts Astronomy & Space Science

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

by Talvikki Hovatta

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.