Long-term variability of Quasars
In Metsähovi a large sample of Active Galactic Nuclei (AGNs) has been monitored at 22 and 37 GHz frequencies for almost 30 years. Using this large data set it is possible to study the long term variability behaviour of AGNs. The entire monitoring sample includes over 100 sources which are monitored on a regular basis on average once in a month. Our monitoring project provides us with a large number of flux curves which span over two decades. An example of a such flux curve at 37 GHz is shown below. Many sort of variability studies can be made using the flux curves as the basis.
In our recent work we have used a sample of 80 sources and studied the variability time scales in these sources. By studying the time scales we gain understanding in how often the sources have flares and how long these outbursts typically last. We can combine this information with theoretical models and study the differences between the source classes. We found out that flares in these sources happen quite rarely, only once in every 4-6 years. Also the duration of flares is long, of the order of 2 years at 37 GHz. The results show that even 10 years of monitoring is not long enough time to reveal the complete variability behaviour in some sources and long-term monitoring is essential for understanding the sources. For more details see: 2007A&A...469..899H and 2008A&A...488..897H.
In addition to the time scales we can study individual flares in these sources. This way it is possible to study the flare evolution from high radio frequencies to lower frequencies and how it corresponds to current theoretical models. We can calculate statistics about the general properties of the flares such as the duration and typical flux of a flare. These studies require dense monitoring and well-sampled flux curves. In Metsähovi we have the largest database at the higher radio frequencies, which enables us to do such analysis for a large sample of sources. For more details see: 2008A&A...485..51H