The shallow phase of X-ray afterglows

TL;DR

This paper explains the flat decay phase in early X-ray afterglows of Gamma Ray Bursts as prolonged central engine activity producing shells with decreasing Lorentz factors, leading to observable breaks and diverse light curve behaviors.

Contribution

It introduces a model where late internal dissipation and external shocks combine to produce the complex X-ray and optical afterglow light curves observed.

Findings

The plateau ends when Lorentz factor drops below 1/θ_j.

X-ray and optical light curves can behave differently due to combined emission components.

Breaks in light curves correspond to Lorentz factor evolution and viewing geometry.

Abstract

We propose that the flat decay phase in the first 100-10,000 seconds of the X-ray light curve of Gamma Ray Bursts can be interpreted as prolonged activity of the central engine, producing shells of decreasing bulk Lorentz factors Gamma. The internal dissipation of these late shells produces a continuous and smooth emission, usually dominant in X-rays and sometimes in the optical. When Gamma of the late shells is larger than 1/theta_j, where theta_j is the jet opening angle, we see only a portion of the emitting surface. Eventually, Gamma becomes smaller than 1/theta_j, and the entire emitting surface is visible. When Gamma=1/theta_j there is a break in the light curve, and the plateau ends. During the plateau phase, we see the sum of the "late-prompt" emission (due to late internal dissipation), and the "real afterglow" emission (due to external shocks). A variety of different optical and X-ray light curves is possible, explaining why the X-ray and the optical light curves often do not track each other, and why they often do not have simultaneous breaks.