Particle acceleration in radio galaxies with flickering jets: GeV electrons to ultrahigh energy cosmic rays

TL;DR

This paper presents a semi-analytic model showing how jet variability in radio galaxies influences particle acceleration, resulting in observable spectral features and implications for ultrahigh energy cosmic ray origins.

Contribution

It introduces a novel model linking jet variability to particle spectra and UHECR production, highlighting spectral signatures of variability and a new proxy electron concept.

Findings

Jet variability causes spectral hardening features.

The UHECR spectrum cutoff is extended by jet power variation.

Proposes GeV electrons as proxies for escaping UHECRs.

Abstract

Variability is a general property of accretion discs and their associated jets. We introduce a semi-analytic model for particle acceleration and radio jet/lobe evolution and explore the effect of Myr timescale jet variability on the particles accelerated by an AGN jet. Our work is motivated by the need for local powerful ultrahigh energy cosmic ray (UHECR) sources and evidence for variability in AGN and radio galaxies. Our main results are: i) UHECR and nonthermal radiative luminosities track the jet power but with a response set by the escape and cooling times, respectively; ii) jet variability produces structure in the electron, synchrotron and UHECR spectra that deviates from that produced for a constant jet power - in particular, spectral hardening features may be signatures of variability; iii) the cutoff in the integrated CR spectrum is stretched out due to the variation in jet power (and, consequently, maximum CR energy). The resulting spectrum is the convolution of the jet power distribution and the source term. We derive an approximate form for a log-normal distribution of powers; iv) we introduce the idea of $\sim 10$ GeV 'proxy electrons' that are cooling at the same rate that UHECRs of rigidity 10 EV are escaping from the source, and determine the corresponding photon frequencies that probe escaping UHECRs. Our results demonstrate the link between the history of an astrophysical particle accelerator and its particle contents, nonthermal emission and UHECR spectrum, with consequences for observations of radio galaxies and UHECR source models.