Unraveling the Origins of GRB X-ray Plateaus through a Study of X-ray Flares

TL;DR

This study investigates the properties of X-ray flares in gamma-ray bursts, revealing similar behaviors regardless of the presence of a plateau, and supports the idea that many GRBs have low Lorentz factors around tens.

Contribution

It provides observational evidence that flare properties are consistent across GRBs with and without X-ray plateaus, supporting models with low Lorentz factors.

Findings

Flare properties are similar in GRBs with and without plateaus.

The flare width to peak time ratio is approximately 1.

Results support low Lorentz factor models (~tens) for many GRBs.

Abstract

The X-ray light curves of gamma-ray bursts (GRBs) display complex features, including plateaus and flares, that challenge theoretical models. Here, we study the properties of flares that are observed in the early afterglow phase (up to a few thousand of seconds). We split the sample into two groups: bursts with and without X-ray plateau. We find that the distributions of flare properties are similar in each group. Specifically, the peak time ($t_{\rm pk}$) of the flares and the ratio of the flare width to the flare peak time ($w/t_{\rm pk}$) which is found to be $\approx 1$, regardless of the presence of a plateau. We discuss these results in view of the different theoretical models aimed at explaining the origin of the plateau. These results are difficult to explain by viewing angle effects or late-time energy injection, but do not contradict the idea that GRBs with X-ray plateau have low Lorentz factor, of the order of tens. For these GRBs, the dissipation processes that produce the flares naturally occur at smaller radii compared to GRBs with higher Lorentz factors, while the flares maintain a similar behaviour. Our results therefore provide an independent support for the idea that many GRBs have a Lorentz factor of a few tens rather than a few hundreds.