Studying X-ray spectra from large-scale jets of Fanaroff–Riley type I radio galaxies: Application of shear particle acceleration

¹ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physics Science and Technology, Guangxi University, Nanning 530004, China
² Max Planck Institute for Plasma Physics, Boltzmannstraße 2, D-85748 Garching, Germany
³ Institute for Theoretical Physics, University of Heidelberg, Philosophenweg 12, D-69120 Heidelberg, Germany

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Received: 30 December 2025
Accepted: 11 March 2026

Abstract

Shear acceleration is a promising candidate mechanism for particle acceleration in extragalactic relativistic jets. For this study, we explored the application of the shear acceleration model to 17 X-ray-bright jet regions in the large-scale jets of Fanaroff–Riley (FR) type I radio galaxies. We studied the jet properties by fitting the multiwavelength spectral energy distributions (SEDs) in a leptonic framework including synchrotron radiation and inverse-Compton scattering off the cosmic microwave background photons. In order to improve spectral modeling, we analyzed Fermi-LAT data for four sources and reanalyzed archival data of Chandra on three X-ray-bright jet regions. We show that synchrotron radiation from a second, shear-accelerated electron population reaching multi-TeV energies satisfactorily models the X-ray SEDs. We explored three different velocity profiles, including linearly decreasing, power-law, and Gaussian profiles and we find that the inferred jet spine velocities are significantly dependent on the choice of velocity profile. The derived magnetic field strengths range from a few to several tens of μG, and the required power in nonthermal particles is well below the Eddington constraint. For M 87, we find that the summed emission from all jet knots is comparable to H.E.S.S. low-state flux, suggesting that the large-scale jet may play a dominant role for the persistent very high energy emission. A comparison with previous results for FR II jets shows that FR I jets tend to have stronger magnetic fields but lower total electron energy. The larger shear viscosity (slower flows and softer shear electron spectral indices) of FR I jets may imply enhanced entrainment and stronger jet-environment interactions compared to FR II jets. Finally, we discuss the potential contribution of FR I jets to ultra-high-energy cosmic rays.