Duration of the active phase in episodic radio galaxies

Determine the duration of the active phase of AGN jet activity in radio galaxies that undergo multiple episodes of jet launching (for example, restarted radio galaxies and double-double radio galaxies), to inform assessments of nuclear activity duty cycles.

Background

The paper discusses that some radio galaxies do not exhibit a single continuous episode of jet activity, but instead show evidence for multiple, distinct episodes separated by quiescent periods. In such systems, inner lobes trace recent activity while outer lobes retain relic signatures of past activity, observable especially at low radio frequencies due to the long synchrotron lifetimes of low-energy electrons.

Understanding how long a single active phase persists is central to characterizing AGN duty cycles. In J1007+3540, the authors measure radiative ages for different components (inner lobes ~140 Myr; outer lobe/backflow ~240–260 Myr), highlighting episodic behavior but not resolving the general question of the typical duration of an active phase across episodic radio galaxies. Establishing these timescales would link jet energetics, environmental influences, and AGN fueling histories.

References

For such radio sources, how long does the active phase last? This is one of the unanswered questions in AGN studies.

— Probing AGN duty cycle and cluster-driven morphology in a giant episodic radio galaxy  (2601.14219 - Kumari et al., 20 Jan 2026) in Section 1, Introduction

The simplest explanation for the pattern seen in multi-epoch radio sources taken as a whole is that either the jet power and/or the duration of the on-phase reduces with each successive episode. Whether or not this pattern is a feature of most, if not all, multi-epoch radio source will need a larger sample of TDRGs and QDRGs.

— J023721.13$-$010528.5: A Giant $S$-shaped Radio Galaxy Showing Four Episodes of Jet Activity  (2609.08506 - Khadekar et al., 8 Sep 2026) in Section 4, subsection “QDRG J023721.13−010528.5: Comparison with DDRGs, TDRGs and S-shaped Radio Galaxies”