Are some breaks in GRB afterglows caused by their spectra ?

Sharp breaks have been observed in the afterglow light curves of several GRBs; this is generally explained by the jet model. However, there are still some uncertainties concerning this interpretation due to the unclear hydrodynamics of jet sideways expansion. Here we propose an alternative explanation to these observed breaks. If we assume that the multiwavelength spectra of GRB afterglows are not made of exact power law segments but their slope changes smoothly, i.e. dβ/dlogν < 0, where β is the spectral index, we find that this fact can very nicely explain the afterglow light curves showing breaks. Therefore we suggest that some breaks in the afterglow light curves may be caused by their curved spectra. The main feature of this interpretation is that the break time is dependent on the observed frequency, while the jet model produces achromatic breaks in the light curves. In addition, it is very important to know the position of the characteristic frequency νc in the multiwavelength spectrum at the time of the break, since it is a further discriminant between our model and the jet model. We find that although the optical light curves of seven GRB afterglows can be well fitted by the model we propose, in fact only one of them (i.e. GRB000926) can be explained in this framework, since for other ones the characteristic frequency νc is either above the optical after the break or below the optical before the break.