X-ray flares, neutrino cooled disks, and the dynamics of late accretion in GRB engines

TL;DR

This study analyzes X-ray flares in gamma-ray bursts, linking their luminosity decline to changes in the accretion disk state, and concludes that intrinsic engine variability, not external interactions, causes the flares.

Contribution

It connects the timing and luminosity of X-ray flares to the transition between neutrino-cooled and non-cooled accretion disks in GRB engines, proposing a new explanation for flare origins.

Findings

X-ray flare luminosity declines as a power-law with index ~-1.5.

The transition from thin to thick disk occurs at the prompt-flare phase boundary.

External interactions cannot account for the observed flares.

Abstract

We compute the average luminosity of X-ray flares as a function of time, for a sample of 10 long-duration gamma-ray burst afterglows. The mean luminosity, averaged over a timescale longer than the duration of the individual flares, declines as a power-law in time with index ~-1.5. We elaborate on the properties of the central engine that can produce such a decline. Assuming that the engine is an accreting compact object, and for a standard conversion factor between accretion rate and jet luminosity, the switch between a neutrino-cooled thin disk and a non-cooled thick disk takes place at the transition from the prompt to the flaring phase. We discuss the implications of this coincidence under different scenarios for the powering of the GRB outflow. We also show that the interaction of the outflow with the envelope of the progenitor star cannot produce flares out of a continuous relativistic flow, and conclude that it is the dynamics of the disk or the jet-launching mechanism that generates an intrinsically unsteady outflow on timescales much longer than the dynamical timescale of the system. This is consistent with the fact that X-ray flares are observed in short-duration GRBs as well as in long-duration ones.