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Sharp breaks have been observed in the afterglow light curves of severalGRBs; this is generally explained by the jet model. However, there are stillsome uncertainties concerning this interpretation due to the unclearhydrodynamics of jet sideways expansion. Here we propose an alternativeexplanation to these observed breaks. If we assume that the multiwavelengthspectra of GRB afterglows are not made of exact power law segments but theirslope changes smoothly, i.e. $d\beta/d log\nu<0$, where $\beta$ is the spectralindex, we find that this fact can very nicely explain the afterglow lightcurves showing breaks. Therefore we suggest that some breaks in the afterglowlight curves may be caused by their curved spectra. The main feature of thisinterpretation is that the break time is dependent on the observed frequency,while the jet model produces achromatic breaks in the light curves. Inaddition, it is very important to know the position of the characteristicfrequency $\nu_{c}$ in the multiwavelength spectrum at the time of the break,since it is a further discriminant between our model and the jet model. We findthat although the optical light curves of seven GRB afterglows can be wellfitted by the model we propose, in fact only one of them (i.e. GRB000926) canbe explained in this framework, since for other ones the characteristicfrequency $\nu_{c}$ is either above the optical after the break or below theoptical before the break.Comment: Latex, revised version accepted by A&

Are some breaks in GRB afterglows caused by their spectra?