A dust scattering model was recently proposed to explain the shallow X-ray decay ( plateau ) observed prevalently in Gamma-Ray Burst ( GRB ) early afterglows .
In this model the plateau is the scattered prompt X-ray emission by the dust located close ( about 10 to a few hundred pc ) to the GRB site .
In this paper we carefully investigate the model and find that the scattered emission undergoes strong spectral softening with time , due to the model ’ s essential ingredient that harder X-ray photons have smaller scattering angle thus arrive earlier , while softer photons suffer larger angle scattering and arrive later .
The model predicts a significant change , i.e. , \Delta \beta \sim 2 - 3 , in the X-ray spectral index from the beginning of the plateau toward the end of the plateau , while the observed data shows close to zero softening during the plateau and the plateau-to-normal transition phase .
The scattering model predicts a big difference between the harder X-ray light curve and the softer X-ray light curve , i.e. , the plateau in harder X-rays ends much earlier than in softer X-rays .
This feature is not seen in the data .
The large scattering optical depths of the dust required by the model imply strong extinction in optical , A _ { V } \mathrel { \hbox { \hbox to 0.0 pt { \hbox { \lower 4.0 pt \hbox { $ \sim$ } } } \hbox { $ > $ } } } 10 , which contradicts current findings of A _ { V } = 0.1 - 0.7 from optical and X-ray afterglow observations .
We conclude that the dust scattering model can not explain the X-ray plateaus .