The Dust Scattering Model Can Not Explain The Shallow X-ray Decay in GRB Afterglows

TL;DR

The paper critically evaluates the dust scattering model for GRB X-ray afterglows and finds it inconsistent with observed spectral softening, light curve behavior, and optical extinction data, thus refuting its explanatory power.

Contribution

This study provides a detailed analysis showing that the dust scattering model cannot account for the observed properties of GRB X-ray afterglow plateaus.

Findings

Spectral softening during the plateau is minimal in observations.

The model predicts a significant difference in plateau durations between hard and soft X-rays.

Optical extinction implied by the model contradicts observed optical afterglow data.

Abstract

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., $\Delta \b \sim 2 - 3$, 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 \gtrsim $ 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.