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Description
We demonstrate that magnetized mildly relativistic shocks may be efficient particle accelerators capable of accounting for the intense X-ray and gamma-ray emission observed in jets of active galactic nuclei (AGN).
We employ the particle-in-cell (PIC) method to investigate a mildly relativistic shock in a magnetized electron-ion plasma under two magnetic field configurations: an oblique shock with the magnetic field oriented just below the critical angle and a quasi-parallel shock. While the only difference between both simulations is the obliquity of the magnetic field, the results show notable difference between both configurations.
In the oblique configuration, a portion of electrons, which move predominantly along magnetic field lines, is efficiently trapped by the cross-shock potential. Trapped cannot readily escape and instead oscillate within the potential peak at the shock. During this motion, the motional electric field, aided by the ExB drift, accelerates trapped electrons to very high energies. Ions are likewise trapped, albeit within potential troughs, and attain comparable maximum kinetic energies to that of electrons. A portion of trapped ions may escape upstream, undergoing shock surfing acceleration along the shock front.
In the quasi-parallel configuration, elliptically polarized whistler waves are self-generated at the shock and propagate upstream. A fraction of ions escape from the non-stationary downstream region into the upstream, where they are trapped by longitudinal electric waves generated by the oblique propagation of whistler waves. This results in efficient heating to relativistic temperatures. Additionally, several plasma instabilities develop, including the modified two-stream instability (MTSI) and the parametric decay instability (PDI), further contributing to enhanced turbulence and heating.