Limits on diffusive shock acceleration in supernova remnants in the presenceof cosmic-ray streaming instability and wave dissipation

Abstract

The instability in the cosmic-ray precursor of a supernova shock moving in interstellar medium is studied. The level of magnetohydrodynamic turbulence in this region determines the maximum energy of particles accelerated by the diffusive shock acceleration mechanism. A high efficiency of cosmic ray acceleration is accepted, and the consideration is not limited by the case of weak turbulence. It is assumed that Kolmogorov-type nonlinear wave interactions together with the ion-neutral collisions restrict the amplitude of the random magnetic field. As a result, the maximum energy of accelerated particles strongly depends on the age of the supernova remnant. The maximum energy can be as high as ${\sim}10^{17}Z$ eV in young supernova remnants and falls to about ${\sim}10^{10}Z$ eV at the end of the Sedov stage. Thus the standard estimate of maximum particle energy based on the Bohm limit calculated for the interstellar magnetic field strength is not justified in this case. This finding may explain why supernova remnants with age of more than a few thousand years are not prominent sources of very high energy gamma-rays.