Search for a command to run...
The radio emission observed from active galactic nuclei (AGNs) is emitted in their relativistic jets. The emission is highly time-variable, but the longest timescales can be of the order of years or even decades. This poses a challenge on the characterisation of this variability as long-term monitoring is required. Understanding the variability on all timescales is important in establishing the connections between the relativistic jet and the central engine. In this doctoral thesis, the long-term radio variability of AGNs and its connection to the central engine has been analysed. To fully characterise the time-variability of AGNs in the radio domain, decades of observations are required due to the slow nature of the variability in the emission. The aim of this thesis is to estimate whether over 40 years of observations observed at the Aalto University Metsähovi Radio Observatory in 37 GHz is enough to constrain the characteristic timescales of variability and if these timescales can be related to the central supermassive black hole and its accretion disk. Out of 123 analysed sources, only 11 were found to have sufficiently long light curves that are a good characterisation of the source variability over long timescales. This result demonstrates how important continuous radio monitoring programmes are in understanding the relativistic jets of AGNs. Through analysis of these light curves, caveats in the typically used methods were also discovered and reported, thus improving the understanding of the general astronomy community on how to implement the methods and of what issues one should be aware of. Through further analysis of a smaller sample of sources, the characteristic timescales were connected to the durations of the observed outbursts as well as their separations. Using these connections, a well-known link between the central engine and X-ray variability was tested on the data to understand whether this connection could extend to the radio domain. Indeed, as a preliminary result, the characteristic timescales were found to correlate with the ratio between the mass of the central supermassive black hole and the mass accretion rate. In addition, the rise times of the flares were used to analyse the width of the jet at the radio core to test for connections between central engine parameters and the speed of the jet flow. Preliminary evidence was found for a weak positive association between the flare rise times and the bulk Lorentz factor.