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Astro Seminar

Extreme physics in radio-loud AGN: jets, shocks, flickering and the highest energy particles in nature

Speaker: James Matthews (Cambridge)
Date: Wednesday 9 March 2022
Time: 14:00
Venue: Zoom

Both cosmic rays and AGN jets were discovered roughly a century ago, and, despite spectacular progress in that period, we are still far from a complete understanding of either of these extreme phenomena. In this talk, I will explore the links between the two, focusing particularly on particle acceleration in astrophysical jets and the origin of ultrahigh energy cosmic rays (UHECRs); UHECRs are protons and nuclei striking our atmosphere with energies extending beyond 1e20 eV. I will discuss ways in which particles can be accelerated to such extreme energies, focusing particularly on diffusive shock acceleration and the self-regulated cosmic ray acceleration process. Aided by hydrodynamic simulations, I will show that shocks can be formed in backflows in radio galaxies and that these shocks can accelerate particles to ultrahigh energy. I will then discuss a model in which 'dormant’ radio galaxies such as Centaurus A and Fornax A act as slowly-leaking UHECR reservoirs. These sources may also be able to explain the observed UHECR arrival directions, particularly if we allow for time-dependence in their jet power and scattering off nearby magnetic structures; in fact, the UHECR signal we observe may merely be an “echo” of past AGN jet activity within ~20 Mpc. Finally, I will describe new work in which we study how a flickering jet power affects the particles accelerated by the jet and the morphology of the kpc-scale radio lobe. I will also discuss observational applications, highlighting the importance of flickering for jet power inference, the radio-loud/radio-quiet dichotomy and our understanding of radio emission in optically bright quasars.