MNRAS 000,1–17 (2020) Preprint 15 March 2021 Compiled using MNRAS LATEX style file v3.0 Particle acceleration in radio galaxies with flickering jets: GeV electrons to ultrahigh energy cosmic rays James H. Matthews1¢ and Andrew M. Taylor2 1Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK 2DESY, D-15738 Zeuthen, Germany Accepted 2021 March 10. Received 2021 March 9; in original form 2020 December 11 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 ∼ 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. Key words: cosmic rays – acceleration of particles – galaxies: jets – galaxies: active – magnetic fields – radiation mechanisms: non-thermal. 1 INTRODUCTION The accretion disc is intimately connected to the outflows pro- duced by the system, which can take the form of winds or relativistic Multi-wavelength variability in the observed fluxes of accreting jets. If the jets are launched by the Blandford & Znajek(1977) mech- sources is observed on a range of timescales. This flux variability anism, then for a black hole (BH) with gravitational radius A6, the has a number of near-universal characteristics. Often, the variability power of the jet depends on the BH spin parameter (0∗) and mag- is consistent with “flicker” noise, also known as 1/ 5 noise or pink 2 netic flux (Φ퐵) threading the event horizon as & / 2¹0∗Φ퐵/A6º . noise because the power spectrum follows a 1/ 5 slope that is steeper BZ The precise relationship between the accretion rate, <¤ , and jet power than white noise but shallower than red. A pedagogical discussion of is complicated; the production of powerful jets might require spe- flicker noise in astronomy is given by Press(1978). A linear ‘rms- cial conditions such as a magnetically arrested disk (MAD) state flux relation’ has been observed in X-ray binaries (Uttley & McHardy (Narayan et al. 2003; Tchekhovskoy et al. 2011; Liska et al. 2018) 2001; Uttley et al. 2005; Heil et al. 2012), accreting white dwarfs and/or the presence of a disc wind to collimate the flow (e.g. Globus (Scaringi et al. 2012; Van de Sande et al. 2015), blazars (Biteau & 2 arXiv:2103.06900v1 [astro-ph.HE] 11 Mar 2021 & Levinson 2016; Blandford et al. 2019). If we assume that Φ Giebels 2012) and other X-ray AGN (Uttley et al. 2005). A related 퐵 ¹<2A ¤ 2º 0 phenomenon is a log-normal distribution of observed fluxes, seen in is proportional to 6 and that ∗ changes on relatively long 2 both disc-dominated (Gaskell 2004; Alston 2019) and jet-dominated timescales, then &BZ /<2 ¤ . This proportionality is also expected sources (H. E. S. S. Collaboration et al. 2010, 2017; Chevalier et al. on general energetic grounds (e.g. Chatterjee et al. 2019; Davis & 2019; see also Morris, Chakraborty, & Cotter 2019). In addition to Tchekhovskoy 2020). It is therefore reasonable that the jet variability flickering-type variability, sources also undergo episodic bursts of is in some sense a filtered version of the accretion-induced variabil- accretion activity, with prominent examples being the outburst cy- ity, a concept which has been successfully applied to jet modelling in cles of X-ray binaries (Fender et al. 2004), restarting radio galaxies X-ray binaries (Malzac 2014; Malzac et al. 2018). What effect does (Kaiser et al. 2000; Brienza et al. 2018; Konar et al. 2019) and, pos- this jet variability have in AGN and radio galaxies? How do multi- sibly, the optical ‘changing-look’ phenomenon in quasars (LaMassa ple episodes of accretion, which are themselves variable, affect the et al. 2015; MacLeod et al. 2016; Runnoe et al. 2016). jet propagation, feedback and any observable radiative or UHECR signatures? In AGN, there are also longer-term aspects of the accretion pro- ¢ [email protected] cess that can imprint variability in the observed accretion flux and © 2020 The Authors 2 J. H. Matthews and A. M. Taylor the power in the jet. For example, on long timescales (&Myr), the ∼ 300 kpc (Morganti et al. 1999; Croston et al. 2009), which appear accretion rate is likely to be determined by the supply of cold gas to to be connected with different episodes of activity. The merger and the central region of the galaxy. In the chaotic cold accretion model gas fuelling history of the sources is complex in both Fornax A (e.g. proposed by Gaspari(2016), the accretion rate on to the BH is pre- Iyomoto et al. 1998; Mackie & Fabbiano 1998; Iodice et al. 2017) dicted to follow a log-normal distribution with a pink/flicker noise and Centaurus A (e.g. Stickel et al. 2004; Neff et al. 2015). In a more power spectrum, just as is observed on shorter timescales in accretion general sense, it is clear that there are many aspects of radio galaxies discs. Simulations by Yang & Reynolds(2016) also show flickering that are time integrated – for example, the total energy input into the type variability in the jet power caused by self-regulated accretion surroundings, or the total energy stored in synchrotron-emitting elec- on to the central AGN, with jet power consequently varying over a trons – and variability creates a disconnect between these integrated large range, (¡ 2 dex; see also Beckmann et al. 2019). Long-term quantities and the instantaneous jet properties. variability in jet power therefore seems inevitable from a fuelling In this paper, our aims are threefold: (i) to introduce a numerical perspective. method capable of modelling, in a simple parameterised fashion, Astrophysical jets accelerate nonthermal particles (see Matthews the morphology, radiation and UHECR signatures from radio galax- et al. 2020 for a review), which radiate as they interact with magnetic ies with variable jet power; (ii) to study the effect of jet variability fields or radiation. Our primary way of learning about radio galaxies, on observational signatures such as the synchrotron luminosity and is, as the name suggests, through radio emission from synchrotron- broadband spectral energy distribution; (iii) to study the acceleration emitting electrons. As well as these nonthermal electrons, AGN jets and escape of UHECRs in radio galaxies with variable jet powers. can accelerate high-energy protons and ions, which we refer to as In our modelling, we focus on jets with a flicker noise power spec- cosmic rays (CRs). The origin of the highest energy CRs, known as trum and a log-normal power distribution. We make a number of ultrahigh energy cosmic rays (UHECRs), is not known. The maxi- further simplifying assumptions and the model is unlikely to pro- mum energy attainable in a particle accelerator is given by the Hillas vide quantitative matches with real astrophysical sources. Indeed, energy 퐸퐻 = /4V퐵', where V = D/2 is the characteristic velocity, our approach is mostly heuristic – we aim to demonstrate some key 퐵 is the magnetic field strength and ' is the size of the accelera- principles regarding particle acceleration in variable jets that can tion region. The Hillas energy is a general constraint that can be be used to study UHECR and electron acceleration. We begin by understood in terms of moving a particle of charge /4 a distance describing our method (section 2) and present the results from a sin- ' through an optimally arranged −u × H electric field. The Hillas gle simulation in section 3. In section 4, we introduce the concept criterion states that any accelerator must be a factor V−1 larger than of ‘proxy electrons’, before discussing extragalactic CR propagation, the Larmor radius of the highest energy particles it accelerates, i.e. observational applications and limitations of the model. We conclude −1 '¡V '6 ¹퐸퐻 º, where '6 denotes the Larmor radius. Calculating in section 5. 퐸퐻 with some characteristic numbers for radio galaxies reveals they are one of the few sources capable of reaching ¡EeV energies and accelerating UHECRs. For this reason, they have long been discussed 2 A SIMPLE VARIABLE JET AND PARTICLE INJECTION as potential UHECR sources (e.g. Hillas 1984; Norman et al. 1995; MODEL Hardcastle et al. 2009; Eichmann et al. 2018; Matthews et al. 2018, 2019a), along with other classes of AGN jets and alternative sources We begin by introducing our model for the evolution of a flickering jet such as gamma-ray bursts (GRBs), starburst winds and cluster-scale and the nonthermal particles it accelerates.