Duty-cycle and energetics of remnant radio-loud AGN

TL;DR

This paper introduces a new method combining spectral ageing and dynamical models to accurately determine the energetics and duty-cycles of remnant radio-loud AGNs, exemplified by the detailed study of B2 0924+30.

Contribution

The paper presents a novel approach that constrains the energetics and duty-cycles of dying AGNs by integrating spectral and dynamical modeling, improving upon previous methods.

Findings

B2 0924+30 spent approximately 50 Myr in active phase.

The source's quiescent phase lasted about 28 Myr.

The jet kinetic power is estimated at roughly 3.6×10^{37} W.

Abstract

Deriving the energetics of remnant and restarted active galactic nuclei (AGNs) is much more challenging than for active sources due to the complexity in accurately determining the time since the nucleus switched-off. I resolve this problem using a new approach that combines spectral ageing and dynamical models to tightly constrain the energetics and duty-cycles of dying sources. Fitting the shape of the integrated radio spectrum yields the fraction of the source age the nucleus is active; this, in addition to the flux density, source size, axis ratio, and properties of the host environment, provides a constraint on dynamical models describing the remnant radio source. This technique is used to derive the intrinsic properties of the well-studied remnant radio source B2 0924+30. This object is found to spend $50^{+14}_{-12}$ Myr in the active phase and a further $28^{+6}_{-5}$ Myr in the quiescent phase, have a jet kinetic power of $3.6^{+3.0}_{-1.7}\times 10^{37}$ W, and a lobe magnetic field strength below equipartition at the $8\sigma$ level. The integrated spectra of restarted and intermittent radio sources is found to yield a 'steep-shallow' shape when the previous outburst occurred within 100 Myr. The duty-cycle of B2 0924+30 is hence constrained to be $\delta < 0.15$ by fitting the shortest time to the previous comparable outburst that does not appreciably modify the remnant spectrum. The time-averaged feedback energy imparted by AGNs into their host galaxy environments can in this manner be quantified.