On the mechanisms of gamma radiation in the Crab Nebula

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Abstract
Different mechanisms responsible for production of the broad-band non-thermal radiation of the Crab Nebula are discussed. It is shown that the synchrotron radiation, calculated in the framework of the magnetohydrodynamic (MHD) model to describe the spatial distribution of the nebular magnetic field and relativistic electrons, is in good agreement with the observed fluxes from radio frequencies to medium-energy γ-rays, E≤ 1 GeV. The γ-rays observed at higher energies require another radiation mechanism. The inverse Compton (IC) scattering of the relativistic electrons on the synchrotron, ‘dust’ far-IR, and 2.7-K microwave background radiation, is the most effective mechanism for the production of very high energy γ-rays, with E > 100 GeV. The detailed study of the IC radiation spectra shows that although the absolute fluxes of the IC γ-rays are sensitive to the average magnetic field in the nebula, the shape of the IC γ-ray spectrum is rather stable to the basic parameters of the nebula. We discuss also the possible role of bremsstrahlung of the relativistic electrons in the Crab Nebula. Although the mean gas density in the nebula, ####g, is insufficient to expect a significant contribution from bremsstrahlung to the flux of high-energy γ-rays, the effective gas density neff for interactions of relativistic particles with the ambient gas could be much higher than ####g, if the relativistic particles were partially captured in the dense filaments in the Crab Nebula. In this case the bremsstrahlung could play a principal role in the formation of the radiation spectrum of the Crab in the energy region 1–100 GeV. So far there is no information on the acceleration of relativistic protons in the Crab. The upper limits of π0-decay γ-ray fluxes, based on consideration of the energy balance between the magnetic field and relativistic particles in the nebula, show that noticeable contribution of these γ-rays may be expected only at energies above 10 TeV.

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