Testing an unifying view of GRB afterglows

TL;DR

This paper proposes a unifying model for GRB afterglows that combines standard forward shock emission with an additional component, explaining diverse observational features and spectral breaks in optical and X-ray light curves.

Contribution

It introduces a combined model for GRB afterglows that accounts for complex light curve behaviors and spectral features, challenging the standard external shock model.

Findings

Good agreement with observed light curves despite diversity

Detection of spectral breaks in 7 GRBs within XRT band

Optical to X-ray SED evolution consistent with the model

Abstract

Four years after the launch the Swift satellite the nature of the GRBs broadband afterglow behaviour is still an open issue. The standard external shock fireball model cannot easily explain the combined temporal and spectral properties of Optical to X-ray afterglows. We analysed the rest frame de-absorbed and K-corrected Optical and X-ray light curves of a sample of 33 GRBs with known redshift and optical extinction at the host frame. We modelled their broadband behaviour as the sum of the standard forward shock emission due to the interaction of a fireball with the circum-burst medium and an additional component. This description provides a good agreement with the observed light curves despite their complexity and diversity and can also account for the lack of achromatic late times jet breaks and the presence of chromatic breaks in several GRBs lightcurves. In order to test the predictions of such modelling we analysed the X-ray time resolved spectra searching for possible spectral breaks within the observed XRT energy band, finding 7 GRBs showing such a break. The Optical to X-Ray SED evolution of these GRBs are consistent with what expected by our interpretation.