Episodic radio galaxies J0116$-$4722 and J1158+2621: can we constrain the quiescent phase of nuclear activity?

TL;DR

This study investigates the life-cycle of episodic radio galaxies through multifrequency observations, constraining active and quiescent phases, and finds a high incidence of core variability potentially linked to episodic activity.

Contribution

It provides new constraints on the durations of active and quiescent phases in episodic FR II radio galaxies using spectral and radiative analysis.

Findings

Quiescent phases can be as short as hotspot fading time or up to tens of Myr.

None of the studied sources have quiescent phases as long as the previous active phase.

A higher proportion of cores show variability compared to normal FR II radio galaxies.

Abstract

We present multifrequency radio observations of two well known episodic FR\,II radio galaxies (J0116$-$4722 and J1158+2621) and a detailed investigation of the life-cycle of episodic radio galaxies from their spectral and radiative properties. Combining our previous work with the present results, we either constrain or place very good limits on the active and quiescent phases of a small sample of episodic FR\,II radio galaxies. The duration of the quiescent phase can be as small as the hotspot fading time of the previous episode, and as high as a few tens of Myr; however, for none of the sources in our sample is it close to the duration of the active phase of the previous episode. We also find that for many episodic radio galaxies, the nucleus is variable at radio wavelengths. For our small sample of 7 episodic radio galaxies, we find 4 to have strongly variable cores, a much larger proportion than is generally found in samples of normal FR\,II radio galaxies. Studies with larger samples will be required to establish a statistical association between core variability and episodic radio activity.