Testing the External Shock Model of Gamma-Ray Bursts using the Late-Time Simultaneous Optical and X-ray Afterglows

TL;DR

This paper tests the external shock model of gamma-ray burst afterglows by analyzing simultaneous optical and X-ray observations, finding that some events do not conform to the model's predictions during the normal decay phase.

Contribution

It provides observational evidence challenging the classical external shock model by identifying outliers in the decay index relation during the GRB afterglow phase.

Findings

4 out of 14 GRBs violate the model at >3σ level

Some afterglows do not follow the predicted decay index relation

The classical model may need modifications to explain all observations

Abstract

We study the ``normal'' decay phase of the X-ray afterglows of gamma-ray bursts (GRBs), which follows the shallow decay phase, using the events simultaneously observed in the R-band. The classical external shock model -- in which neither the delayed energy injection nor time-dependency of shock micro-physics is considered -- shows that the decay indices of the X-ray and R-band light curves, $\alpha_{\rm X}$ and $\alpha_{\rm O}$, obey a certain relation, and that in particular, $\alpha_{\rm O}-\alpha_{\rm X}$ should be larger than -1/4 unless the ambient density increases with the distance from the central engine. For our selected 14 samples, we have found that 4 events violate the limit at more than the 3$\sigma$ level, so that a fraction of events are outliers of the classical external shock model at the ``normal'' decay phase.