ON THE MECHANISMS OF GAMMA-RADIATION IN THE CRAB-NEBULA

Different mechanisms responsible for production of the broad-band non- thermal radiation of the Crab Nebula are discussed. It is shown that t he synchrotron radiation, calculated in the framework of the magnetohy drodynamic (MHD) model to describe the spatial distribution of the neb ular magnetic field and relativistic electrons, is in good agreement w ith the observed fluxes from radio frequencies to medium-energy gamma- rays, E less than or equal to 1 GeV. The gamma-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 effectiv e mechanism for the production of very high energy gamma-rays, with E > 100 GeV. The detailed study of the IC radiation spectra shows that a lthough the absolute fluxes of the IC gamma-rays are sensitive to the average magnetic field in the nebula, the shape of the IC gamma-ray sp ectrum is rather stable to the basic parameters of the nebula. We disc uss also the possible role of bremsstrahlung of the relativistic elect rons in the Crab Nebula. Although the mean gas density in the nebula, <(n)over bar(g)>, is insufficient to expect a significant contribution from bremsstrahlung to the flux of high-energy gamma-rays, the effect ive gas density n(eff) for interactions of relativistic particles with the ambient gas could be much higher than <(n)over bar(g)>, if the re lativistic particles were partially captured in the dense filaments in the Crab Nebula. In this case the bremsstrahlung could play a princip al 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 acceler ation of relativistic protons in the Crab. The upper limits of pi(0)-d ecay gamma-ray fluxes, based on consideration of the energy balance be tween the magnetic field and relativistic particles in the nebula, sho w that noticeable contribution of these gamma-rays may be expected onl y at energies above 10 TeV.