Three episodes of jet activity in the FRII radio galaxy B0925+420

TL;DR

This paper reports the discovery of a rare 'Triple-Double' radio galaxy with three pairs of aligned lobes, indicating multiple episodes of jet activity, and models their dynamical evolution considering buoyant displacement and bow shocks.

Contribution

First identification of a 'Triple-Double' radio galaxy, expanding understanding of episodic jet activity and modeling the lobes' evolution in different environmental conditions.

Findings

Discovery of a third pair of lobes in B0925+420.

Inner lobes are consistent with buoyant displacement.

Middle lobes may be bow shocks within outer lobes.

Abstract

We present Very Large Array images of a "Double-Double Radio Galaxy", a class of objects in which two pairs of lobes are aligned either side of the nucleus. In this object, B0925+420, we discover a third pair of lobes, close to the core and again in alignment with the other lobes. This first-known "Triple-Double" object strongly increases the likelihood that these lobes represent mutiple episodes of jet activity, as opposed to knots in an underlying jet. We model the lobes in terms of their dynamical evolution. We find that the inner pair of lobes is consistent with the outer pair having been displaced buoyantly by the ambient medium. The middle pair of lobes is more problematic - to the extent where an alternative model interpreting the middle and inner "lobes" as additional bow shocks within the outer lobes may be more appropriate - and we discuss the implications of this on our understanding of the density of the ambient medium.