Gamma-Ray Burst Afterglow emission with a decaying magnetic field

In models for gamma ray burst afterglows, it is normally assumed that the external shock strongly amplifies the magnetic field and that this field maintains a steady value throughout the shocked region. We discuss the effects of modifying this (probably simplistic) assumption. The observations are incompatible with a post-shock field that decays too rapidly. However if the field pervaded only a few percent of the total thickness of the shocked shell (and the electrons undergo only inverse Compton losses in the remainder) the models would be more compatible with the high external densities expected in star-forming regions. Afterglow observations in all wavebands, especially at radio wavelengths, could help to pin down the field structure.