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Description
Based on direct numerical modeling, it is shown that pulsar wind nebulae (PWN) with a double X-ray torus are capable of accelerating very high energy protons. These objects (with well known Vela nebula as the prototype) are distinguished by a special structure of MHD flows of strongly magnetized plasma. This special structure of the nebula allows for longtime confinement of high-energy particles which are the subject of combined acceleration mechanisms. The mechanisms include acceleration on shear and counter-streaming MHD flows, as well as Type I Fermi acceleration. The injected CR particle trajectories were directly followed in the simulated PWN structure. The maximum energies that protons can gain in the body of a compact double-torus nebula exceeds 100 TeV. The acceleration process allows 5-6% of protons (in terms of the total number of particles involved) to increase their Lorentz factor from $10^4$ to ≳ $5\times 10^5$ in just 10-30 years. This efficiency can be maintained as long as the nebula retains its double-torus X-ray morphology, which can lasts from a few thousand to several hundred thousand years, depending on parameters of the pulsar and the external environment in which the pulsar moves.
