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The origin of the extended X-ray emission in the large-scale jets of activegalactic nuclei (AGNs) poses challenges to conventional models of accelerationand emission. Although the electron synchrotron radiation is considered themost feasible radiation mechanism, the formation of the continuous large-scaleX-ray structure remains an open issue. As astrophysical jets are expected toexhibit some turbulence and shearing motion, we here investigate the potentialof shearing flows to facilitate an extended acceleration of particles andevaluate its impact on the resultant particle distribution. Our treatmentincorporates systematic shear and stochastic second-order Fermi effects. Weshow that for typical parameters applicable to large-scale AGN jets, stochasticsecond-order Fermi acceleration, which always accompanies shear particleacceleration, can play an important role in facilitating the whole process ofparticle energization. We study the time-dependent evolution of the resultantparticle distribution in the presence of second-order Fermi acceleration, shearacceleration, and synchrotron losses using a simple Fokker--Planck approach andprovide illustrations for the possible emergence of a complex (multicomponent)particle energy distribution with different spectral branches. We presentexamples for typical parameters applicable to large-scale AGN jets, indicatingthe relevance of the underlying processes for understanding the extended X-rayemission and the origin of ultrahigh-energy cosmic rays.

astrophysical journal/the astrophysical journal

Particle Acceleration in Mildly Relativistic Shearing Flows: the  Interplay of Systematic and Stochastic Effects, and the Origin of the  Extended High-energy Emission in AGN Jets