Shear Particle Acceleration in Astrophysical Jets
Final Report Abstract
This project focused on the fundamental question: where and how are the charged particles observed in the Universe accelerated to extreme energies? One of the most promising classes of cosmic particle accelerators are Active Galactic Nuclei (AGN) - galaxies harbouring accreting supermassive black hole systems that launch relativistic plasma outflows, or jets. Increasing evidence suggests that these jets commonly exhibit velocity shear, prompting renewed interest in its role in particle acceleration and high-energy emission. Fast shear flows can in principle facilitate energetic particle acceleration by several means. A key process explored in this project is a stochastic Fermi-type acceleration mechanism, whereby particles gain energy via elastically scattering off magnetic inhomogeneities embedded in a shearing background flow. This has been investigated here by studying the expected properties of simulated jets, numerically solving the transport of energetic charged particles in such environments, extending semi-analytical frameworks and developing phenomenological models to facilitate direct comparison with observations. Our main findings show: (1) Kelvin-Helmholtz instabilities naturally give rise to shear layers in relativistic jets, enabling particle acceleration; (2) Shear acceleration efficiently produces power-law particle spectra, with spectral index sensitive to the flow profile; (3) Efficient in-situ acceleration of electrons along the jets provides a natural explanation for the extended, high-energy electron synchrotron emission in AGN jets; (4) Our findings support the possibility that such a mechanism contributes to the production of high-energy neutrino emission from nearby Seyfert AGN; and (5) Shear acceleration offers a viable mechanism or the production of ultra-high-energy cosmic rays, particularly in sources such as Centaurus A. The results significantly advance our conceptual understanding of cosmic particle acceleration and identify shear flows in AGN jets as a key environment for high-energy, multi-messenger phenomena.
Publications
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Active Galactic Nuclei as Potential Sources of Ultra-High Energy Cosmic Rays. Universe, 8(11), 607.
Rieger, Frank M.
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Particle Acceleration in Relativistic Shearing Flows: Energy Spectrum. The Astrophysical Journal, 933(2), 149.
Rieger, Frank M. & Duffy, Peter
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Particle acceleration in shearing flows: the self-generation of turbulent spine-sheath structures in relativistic magnetohydrodynamic jet simulations. Monthly Notices of the Royal Astronomical Society, 519(2), 1872-1880.
Wang, Jie-Shuang; Reville, Brian; Mizuno, Yosuke; Rieger, Frank M. & Aharonian, Felix A.
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Studying X-ray spectra from large-scale jets of FR II radio galaxies: application of shear particle acceleration. Monthly Notices of the Royal Astronomical Society, 525(4), 5298-5310.
He, Jia-Chun; Sun, Xiao-Na; Wang, Jie-Shuang; Rieger, Frank M.; Liu, Ruo-Yu & Liang, En-Wei
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Theory of Gamma-Ray loud AGNs. Proceedings of 7th Heidelberg International Symposium on High-Energy Gamma-Ray Astronomy — PoS(Gamma2022), 002. Sissa Medialab.
Rieger, Frank
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Acceleration of Ultra-high-energy Cosmic Rays in the Kiloparsec-scale Jets of Nearby Radio Galaxies. The Astrophysical Journal Letters, 977(1), L20.
Wang 王, Jie-Shuang 界双; Reville, Brian; Rieger, Frank M. & Aharonian, Felix A.
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Nonlinear aspects of stochastic particle acceleration. Physical Review D, 109(6).
Lemoine, Martin; Murase, Kohta & Rieger, Frank
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The Role of Stochastic Fermi-type Particle Acceleration in the Inner Jets of Active Galactic Nuclei. The Astrophysical Journal, 967(1), 36.
Wang, Jieshuang; Rieger, Frank M. & Mizuno, Yosuke
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Neutrinos from stochastic acceleration in black hole environments. Astronomy & Astrophysics, 697, A124.
Lemoine, Martin & Rieger, Frank
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Synchrotron-limited Particle Acceleration in Relativistic Shearing Flows. The Astrophysical Journal, 988(2), 245.
M. Rieger, Frank & Duffy, Peter
