Particle acceleration in shearing flows: the case for large-scale jets
  X-ray observations of kilo-parsec scale jets indicate that a synchrotron
origin of the sustained non-thermal emission is likely. This requires
distributed acceleration of electrons up to near PeV energies along the jet.
The underlying acceleration mechanism is still unclear. Shear acceleration is a
promising candidate, as velocity-shear stratification is a natural consequence
of the collimated flow of a jet. We study the details of shear acceleration by
solving the steady-state Fokker-Planck-type equation and provide a simple
general solution for trans-relativistic jets for a range of magnetohydrodynamic
turbulent power-law spectra. In general, the accelerated particle population is
a power-law spectrum with an exponential-like cut-off, where the power-law
index is determined by the turbulence spectrum and the balance of escape and
acceleration of particles. Adopting a simple linearly decreasing velocity
profile in the boundary of large-scale jets, we find that the multi-wavelength
spectral energy distribution of X-ray jets, such as Centaurus A and 3C 273, can
be reproduced with electrons that are accelerated up to $\sim$ PeV. In
kpc-scale jets, protons may be accelerated up to $\sim$ EeV, supporting the
hypothesis that large-scale jets are strong candidates for
ultra-high-energy-cosmic-ray sources within the framework of shear
acceleration.
