Jet breaks at the end of the slow decline phase of Swift GRB lightcurves

TL;DR

This paper proposes a double component outflow model to explain the complex chromatic breaks observed in Swift GRB afterglow lightcurves, challenging the traditional single-component energy injection interpretation.

Contribution

It introduces a novel double outflow model with narrow and broad components to explain chromatic breaks in GRB afterglows, expanding the understanding of jet structure.

Findings

The model can produce early jet breaks in X-ray without optical breaks.

It accounts for chromatic breaks by different collimation of outflow components.

Theoretical analysis constrains physical parameters compatible with observations.

Abstract

The Swift mission has discovered an intriguing feature of Gamma-Ray Burst (GRBs) afterglows, a phase of shallow decline of the flux in the X-ray and optical lightcurves. This behaviour is typically attributed to energy injection into the burst ejecta. At some point this phase ends, resulting in a break in the lightcurve, which is commonly interpreted as the cessation of the energy injection. In a few cases, however, while breaks in the X-ray lightcurve are observed, optical emission continues its slow flux decline. This behaviour suggests a more complex scenario. In this paper, we present a model that invokes a double component outflow, in which narrowly collimated ejecta are responsible for the X-ray emission while a broad outflow is responsible for the optical emission. The narrow component can produce a jet break in the X-ray lightcurve at relatively early times, while the optical emission does not break due to its lower degree of collimation. In our model both components are subject to energy injection for the whole duration of the follow-up observations. We apply this model to GRBs with chromatic breaks, and we show how it might change the interpretation of the GRBs canonical lightcurve. We also study our model from a theoretical point of view, investigating the possible configurations of frequencies and the values of GRB physical parameters allowed in our model.