Spectral variation
• The spectral index is nearly constant at the impulsive phase à constant acceleration spectrum
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• The spectral slope on the high frequency side  in the decay phase hardens with time, delay of higher frequency emission à Coulomb collisions
But still we’re not entirely sure, because we do not what extent the observed spectral variation is due to really Coulomb collisions but what if it is partly or even entirely controlled by such acceleration spectrum variation, although such a pattern was never reported, but still not impossible in general. So we examined in more detail. Here The solid line=spectral index, going to smaller number means hardening. The dotted & dahsed = light-curves.