Swift and Fermi observations of X-ray flares: the case of Late Internal Shock

TL;DR

This paper analyzes simultaneous Swift and Fermi observations of GRB X-ray flares, showing they are likely produced by late internal shocks from relativistic outflows, providing insights into the central engine activity.

Contribution

It presents a comprehensive sample of X-ray flares observed simultaneously by Swift and Fermi, and tests the late internal shock model against broadband data.

Findings

X-ray flares are often undetected at optical and GeV energies.

X-ray flares can be produced by late internal shocks at radii of 10^13-10^14 cm.

Flares probe the activity of the GRB central engine.

Abstract

Simultaneous Swift and Fermi observations of gamma-ray bursts (GRBs) offer a unique broadband view of their afterglow emission, spanning more than ten decades in energy. We present the sample of X-ray flares observed by both Swift and Fermi during the first three years of Fermi operations. While bright in the X-ray band, X-ray flares are often undetected at lower (optical), and higher (MeV to GeV) energies. We show that this disfavors synchrotron self-Compton processes as origin of the observed X-ray emission. We compare the broadband properties of X-ray flares with the standard late internal shock model, and find that, in this scenario, X-ray flares can be produced by a late-time relativistic (Gamma>50) outflow at radii R~10^13-10^14 cm. This conclusion holds only if the variability timescale is significantly shorter than the observed flare duration, and implies that X-ray flares can directly probe the activity of the GRB central engine.