Episodic Activity in Active Galactic Nuclei

TL;DR

This paper reviews evidence for episodic activity in active galactic nuclei, especially radio galaxies, discussing observational signatures, time scales, and implications for understanding AGN life cycles.

Contribution

It provides a comprehensive overview of episodic AGN activity, highlighting observational evidence, potential detection methods, and trends in associated phenomena.

Findings

Radio galaxies show multiple lobe pairs indicating episodic activity.

Steep radio spectra suggest older activity cycles detectable at low frequencies.

Higher incidence of H I absorption correlates with episodic activity evidence.

Abstract

There is increasing evidence to suggest that AGN activity may be episodic, with a wide range of possible time scales. Radio galaxies exhibit the most striking examples of episodic activity, with two or three distinct pairs of lobes on opposite sides of the active nucleus. Radio emission from earlier cycles of activity are expected to have steep radio spectra due to radiative losses, and hence be detected more easily at low radio frequencies. Inverse-Compton scattered cosmic microwave background radiation could in prinicple probe even lower Lorentz-factor particles, revealing an older population. We illustrate the time scales of episodic activity by considering different radio galaxies, discuss the possiblity of episodic activity in cluster radio sources, and a possible trend for a high incidence of H{\sc i} absorption in sources with evidence of episodic activity.