Magnetic Field Amplification in Tycho and other Shell-type Supernova Remnants

It is shown that amplification of the magnetic field in supernova remnants (SNRs) occurs in all seven objects where morphological measurements are presently available in the hard X-ray continuum at several keV. For the three archetypical objects (SN 1006, Cas A and Tycho's SNR) to which nonlinear time-dependent acceleration theory has been successfully applied up to now, the global theoretical and the local observational eld strengths agree very well, suggesting that all young SNRs exhibit the amplification effect as a result of very efficient acceleration of nuclear cosmic rays (CRs) at the outer shock. A fourth such case is presented elsewhere in these Proceedings for the remnant of Kepler's SN, with the same conclusion. We consider field amplification as a measure of nuclear CR acceleration. The above results are also used to investigate the time evolution of field amplification in young SNRs. They rather clearly show that the ratio of the magnetic field energy density and the kinetic energy density of gas flow into the shock is of the order of a few percent if the shock speed is high enough, Vs>103 km/s. It implies that the escape of the highest energy nuclear particles from their sources becomes progressively important with age, reducing also the cutoff in the π0-decay gamma-ray emission spectrum with time after the end of the sweep-up phase