Recurrent Activity in Active Galactic Nuclei

TL;DR

This paper reviews evidence for episodic activity in active galactic nuclei (AGN), focusing on radio observations of radio galaxies and quasars, and discusses methods to determine the time scales of recurrent activity.

Contribution

It provides a comprehensive review of observational evidence and methods for studying episodic AGN activity, highlighting the significance of radio structures and spectral ages.

Findings

Double-double radio sources indicate recurrent activity.

Spectral and dynamical ages constrain activity time scales.

Inverse-Compton scattering can reveal older electron populations.

Abstract

There has been a growing body of evidence to suggest that AGN activity, which is powered by mass accretion on to a supermasive black hole, could be episodic, although the range of time scales involved needs to be explored further. The structure and spectra of radio emission from radio galaxies, whose sizes range up to $\sim$5 Mpc, contain information on the history of AGN activity in the source. They thus provide a unique opportunity to study the time scales of recurrent AGN activity. The most striking examples of recurrent activity in radio galaxies and quasars are the double-double or triple-double radio sources which contain two or three pairs of distinct lobes on opposite sides of the parent optical object. Spectral and dynamical ages of these lobes could be used to constrain time scales of episodic activity. Inverse-Compton scattered cosmic microwave background radiation could in principle probe lower Lorentz-factor particles than radio observations of synchrotron emission, and thereby reveal an older population. We review briefly the radio continuum as well as molecular and atomic gas properties of radio sources which exhibit recurrent or episodic activity, and present a few cases of quasars which require further observations to confirm their episodic nature. We also illustrate evidence of episodic AGN activity in radio sources in clusters of galaxies.