Spectral curvature behavior during X-ray flares in GRB afterglow emission

TL;DR

This paper analyzes the spectral curvature of X-ray flares in GRB afterglows using a log-parabolic model, revealing a significant curvature and an anti-correlation between peak energy and curvature, supporting a stochastic acceleration scenario.

Contribution

It introduces the application of the log-parabolic spectral model to X-ray flares in GRB afterglows and demonstrates its statistical superiority over other models.

Findings

X-ray flares have remarkably curved spectra.

The log-parabolic model fits the data better than alternatives.

An anti-correlation between peak energy and curvature is observed.

Abstract

One of the most impressive recent discovery of SWIFT is the evidence that X-ray flares occurring during the GRB afterglows are quite common, being observed in roughly 50% of the afterglows. These X-ray flares range fluences comparable with the GRB prompt emission and could be also repetitive. Several pictures have been proposed on their origin and among them the most accepted regards the internal shock scenario, interpreting the X-ray flares as late time activity of the GRB central engine. We propose to describe the spectral shape of the X-ray flares adopting the same physical model recently used to interpret the GRB prompt emission: the log-parabolic function. In particular, we show that their spectral energy distribution (SED) is remarkably curved, while no significant curvature appears in the underlying X-ray afterglow emission. In addition, the log-parabolic function is statistically favored with respect to other proposed spectral models. By using a time resolved spectral analysis, we show the evolution of the peak energy and the curvature parameters of the SED during the X-ray flares in two of the brightest GRBs afterglows observed by SWIFT. We found that in the X-ray flares there is an anti-correlation between the peak energy and the curvature, as expected in a stochastic acceleration scenario.