Long term radio variability of AGN

TL;DR

This study analyzes over 30 years of radio observations across multiple frequencies for a large sample of AGN, revealing variability patterns, flare characteristics, and predicting future flares relevant for gamma-ray observations.

Contribution

It combines multi-frequency long-term radio data to characterize AGN variability and predicts future flares, highlighting the importance of extended monitoring.

Findings

Characterized flare properties through visual inspection.

Determined variability timescales as a function of frequency.

Predicted sources likely to exhibit large flares in 2008-2009.

Abstract

A large number of AGN have been monitored for nearly 30 years at 22, 37 and 87 GHz in Mets\"ahovi Radio Observatory. These data were combined with lower frequency 4.8, 8.0 and 14.5 GHz data from the University of Michigan Radio Astronomy Observatory, higher frequency data at 90 and 230 GHz from SEST, and supplementary higher frequency data from the literature to study the long-term variability of a large sample of AGN. Both the characteristics of individual flares from visual inspection and statistically-determined variability timescales as a function of frequency and optical class type were determined. Based on past behaviour, predictions of sources expected to exhibit large flares in 2008--2009 appropriate for study by GLAST and other instruments are made. The need for long-term data for properly understanding source behaviour is emphasised.