A lower limit to the magnetic field in Cassiopeia-A

Abstract

The magnetic field strength in the radio-emitting shell of the supernova remnant Cassiopeia-A should be greater than $$8 \times 10^{-5} G$$ if the bremsstrahlung by the electrons responsible for the non-thermal radio emission is not to exceed the upper limits to the gamma-ray emissivity set by the recent observations. This field strength is shown to be too large to be generated by a mere compression of the interstellar field by the supernova shock but must arise due to magnetohydrodynamic instabilities in the expanding shell. Gamma-ray generation through inverse-Compton scattering of photons in the surrounding H II region is also briefly discussed.