2002MNRAS.330..955L

We discuss the afterglow emission from a relativistic jet that is initially in the radiative regime, in which the accelerated electrons are fast-cooling. We note that such a `semiradiative' jet decelerates faster than an adiabatic jet does. We also take into account the effect of strong inverse-Compton scattering on the cooling frequency in the synchrotron component and therefore on the light-curve decay index. We find that there are two kinds of light-curve break for the jet effect. The first is an `adiabatic break', if the electrons become slow-cooling before the jet enters a spreading phase, and the second is a `radiative break', which appears in the contrary case. We then show how a relativistic jet evolves dynamically and derive the overall temporal synchrotron emission in both cases, focusing on the change in the light-curve decay index around the break time. Finally, in view of our results, we rule out two cases for relativistic jets which do not account for the observed light-curve breaks in a few afterglows: (i) an adiabatic jet with strong Compton cooling (Y >1) and with the cooling frequency ν_c locating in the observed energy range; (ii) a radiative jet with a significant fraction of total energy occupied by electrons (ε_e∼1).