X-ray flares from propagation instabilities in long Gamma-Ray Burst jets

TL;DR

This study uses numerical simulations to show that propagation instabilities in long gamma-ray burst jets can produce X-ray flares with properties similar to observed flares, suggesting a new origin for these phenomena.

Contribution

The paper demonstrates that propagation instabilities, not engine variability, can account for moderate X-ray flares in long GRBs, providing a new explanation for their origin.

Findings

Simulated jet fluctuations match observed flare properties.

Propagation instabilities can produce moderate but not strong flares.

Engine variability is necessary for the strongest observed flares.

Abstract

We present a numerical simulation of a gamma-ray burst jet from a long-lasting engine in the core of a 16 solar mass Wolf-Rayet star. The engine is kept active for 6000 s with a luminosity that decays in time as a power-law with index -5/3. Even though there is no short time-scale variability in the injected engine luminosity, we find that the jet's kinetic luminosity outside the progenitor star is characterized by fluctuations with relatively short time scale. We analyze the temporal characteristics of those fluctuations and we find that they are consistent with the properties of observed flares in X-ray afterglows. The peak to continuum flux ratio of the flares in the simulation is consistent with some, but not all, the observed flares. We propose that propagation instabilities, rather than variability in the engine luminosity, are responsible for the X-ray flares with moderate contrast. Strong flares such as the one detected in GRB 050502B, instead, cannot be reproduced by this model and require strong variability in the engine activity.