James H Matthews

Recent anisotropy studies of UHECR data at energies ≳40 EeV have disclosed a correlation of their angular distribution with the extragalactic local structure, specifically with either local starburst galaxies or AGN. Using Monte Carlo simulations taking into account photodisintegration processes, we further explore a framework in which these UHECRs were accelerated by Centaurus A in a recent powerful outburst before being scattered by magnetic fields associated with local, Council of Giant, extragalactic structure. We find that the observed intermediate-scale anisotropies can be accounted for by the Council of Giant structure imposing a response function on the initial outburst of UHECRs from a single source located at Centaurus A’s position. The presence of these local structures create ‘echoes’ of UHECRs after the initial impulse and focusing effects. The strongest echo wave has a lag of ∼20 Myr, comparable to the age of synchrotron-emitting electrons in the giant Centaurus A lobes. Through consideration of the composition of both the direct and echo wave components, we find that the distribution of the light (1 < ln A < 1.5) component across the sky offers exciting prospects for testing the echo model using future facilities such as Auger prime. Our results demonstrate the potential that UHECR nuclei offer, as ‘composition clocks’, for probing propagation scenarios from local sources.

We present two- and three-dimensional hydrodynamic simulations of ∼kpc-scale AGN jets with mean jet powers in the range 1–7 × 1045 erg s−1, in which the jet power varies (through variation of the Lorentz factor) according to a flicker or pink noise power spectrum. We find the morphology and dynamics of the jet–cocoon system depends on the amplitude of the variability with a clear correspondence between the shape of the cocoon and the historical activity. The jet advances quickly during high-power states, whereas quiescent periods instead produce passive periods of inflation resembling Sedov–Taylor blast waves. Periods of high activity preferentially produce hotspots and create stronger backflow as they maximize the pressure gradient between the jet head and cocoon. The variability can also lead to propagating internal shock structures along the jet. Our work suggests that variability and flickering in the jet power has important implications, which we discuss, for observations of radio galaxies, ultrahigh energy cosmic ray acceleration and jet power to luminosity correlations. We explore the link between morphology and fuelling, and suggest that chaotic cold accretion should introduce a relatively small scatter in radio luminosity (∼0.2 dex) and modest imprints on morphology; sources such as Hercules A and Fornax A, which show evidence for more dramatic variability, may therefore require redder power spectra, or be triggered by mergers or other discrete events. We suggest ways to search for jet flickering observationally and propose that radio galaxies may be an important diagnostic of Myr time-scale AGN fuelling, due to their ‘long-term memory’.