Drury et al. (1999) concluded that a major defect of all 'box' models is the basic assumption that all particles gain and lose energy at exactly the same rate. It is clear physically that there are very large fluctuations in the time particles spend in the upstream and downstream regions between shock crossings, and thus correspondingly large fluctuations for energy lost. The effect of these variations will be to smear out the artificially sharp pile-ups predicted by the simple 'box' models. However, the results described above are based simply on the scaling with energy of the various gain and loss processes together with the size of the acceleration region. Thus, they should be relatively robust, and Drury et al. (1999) expect that even if there is no sharp spike, the spectrum will show local enhancements over what it would have been in the absence of the synchrotron or inverse Compton losses in those cases in which this criterion is satisfied.
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