The variability of brightest cluster galaxies at high radio frequencies

TL;DR

This study investigates the high-frequency radio variability of 20 nearby brightest cluster galaxies, revealing significant variability over years, which impacts astrophysical observations and understanding of AGN activity.

Contribution

It provides the first comprehensive analysis of high-frequency variability in brightest cluster galaxies over multi-year timescales using diverse observational data.

Findings

Most galaxies vary significantly over approximately one year.

At least half show 20% peak-to-trough variability on 3-year timescales.

At least a third vary by 60% on 6-year timescales.

Abstract

Variability of a galaxy's core radio source can be a significant consequence of AGN accretion. However, this variability has not been well studied, particularly at high radio frequencies. As such, we report on a campaign monitoring the high radio frequency variability of 20 nearby, cool-core brightest cluster galaxies. From our representative sample, we show that most vary significantly over timescales of approximately one year and longer. Our highest cadence observations are at 15 GHz and are from the Owens Valley Radio Observatory (OVRO). They have a median time interval of 7 days and mostly span between 8 and 13 years. We apply a range of variability detection techniques to the sources' lightcurves to analyse changes on week to decade long timescales. Most notably, at least half of the sources show 20 per cent peak to trough variability on 3 year timescales, while at least a third vary by 60 per cent on 6 year timescales. Significant variability, which is important to studies of the Sunyaev-Zel'dovich Effect in the radio/sub-mm, is therefore a common feature of these sources. We also show how the variability relates to spectral properties at frequencies of up to 353 GHz using data from the Korean VLBI network (KVN), the NIKA2 instrument of the IRAM 30m telescope, and the SCUBA-2 instrument of the James Clerk Maxwell Telescope.