Adiabatic expansion, early x-ray data and the central engine in GRBs

TL;DR

This paper models the adiabatic expansion of residual emission in GRBs to explain early X-ray decay, comparing predictions with observations and suggesting most central engines remain active during this phase.

Contribution

It introduces a new micro-physical model for adiabatic expansion emission in GRBs and compares it with observational data to assess its applicability.

Findings

Only about 20% of bursts match the adiabatic expansion model.

Most GRBs likely have ongoing central engine activity during early X-ray decay.

The model provides specific spectral and temporal predictions for cooling ember emission.

Abstract

The Swift satellite early x-ray data shows a very steep decay in most of the Gamma-Ray Bursts light curves. This decay is either produced by the rapidly declining continuation of the central engine activity or by some left-over radiation starting right after the central engine shuts off. The latter scenario consists of the emission from an "ember" that cools via adiabatic expansion and, if the jet angle is larger than the inverse of the source Lorentz factor, the large angle emission. In this work, we calculate the temporal and spectral properties of the emission from such a cooling ember, providing a new treatment for the micro-physics of the adiabatic expansion. We use the adiabatic invariance of p_{\perp}^2/B (p_{\perp} is the component of the electrons' momentum normal to the magnetic field, B) to calculate the electrons' Lorentz factor during the adiabatic expansion; the electron momentum becomes more and more aligned with the local magnetic field as the expansion develops. We compare the theoretical expectations of the adiabatic expansion (and the large angle emission) with the current observations of the early x-ray data and find that only about 20% of our sample of 107 bursts is potentially consistent with this model. This leads us to believe that, for most bursts, the central engine does not turn off completely during the steep decay of the x-ray light curve; therefore, this phase is produced by the continued rapidly declining activity of the central engine.