Long-term radio variability of AGN: flare characteristics

¹ Metsähovi Radio Observatory, TKK, Helsinki University of Technology, Metsähovintie 114, 02540 Kylmälä, Finland e-mail: tho@kurp.hut.fi
² Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
³ Department of Physics, University of Turku, 20100 Turku, Finland
⁴ Department of Astronomy, University of Michigan, Ann Arbor, MI 48109, USA

Received: 19 March 2008
Accepted: 18 April 2008

Abstract

Aims. We have studied the flare characteristics of 55 AGN at 8 different frequency bands between 4.8 and 230 GHz. Our extensive database enables us to study the various observational properties of flares in these sources and compare our results with theoretical models.

Methods. We visually extracted 159 individual flares from the flux density curves and calculated different parameters, such as the peak flux density and duration, in all the frequency bands. The selection of flares is based on the 22 and 37 GHz data from Metsähovi Radio Observatory and 90 and 230 GHz data from the SEST telescope. Additional lower frequency 4.8, 8, and 14.5 GHz data are from the University of Michigan Radio Observatory. We also calculated variability indices and compared them with earlier studies.

Results. The observations seem to adhere well to the shock model, but there is still large scatter in the data. Especially the time delays between different frequency bands are difficult to study due to the incomplete sampling of the higher frequencies. The average duration of the flares is 2.5 years at 22 and 37 GHz, which shows that long-term monitoring is essential for understanding the typical behaviour in these sources. It also seems that the energy release in a flare is independent of the duration of the flare.