'Jet breaks' and 'missing breaks' in the X-Ray afterglow of Gamma Ray Bursts

TL;DR

This paper analyzes the diverse X-ray afterglow light curves of Gamma-Ray Bursts within the cannonball model, explaining the presence or absence of breaks and their relation to jet geometry and energy, supported by observational data.

Contribution

It demonstrates that all observed X-ray afterglow shapes align with the cannonball model's predictions, clarifying the nature of breaks and missing breaks in GRB afterglows.

Findings

All X-ray afterglow shapes are consistent with the CB model.

In energetic GRBs, breaks are hidden or occur too early to detect.

Spectral and temporal indices differ by half a unit as predicted.

Abstract

The X-ray afterglows (AGs) of Gamma-Ray Bursts (GRBs) and X-Ray Flashes (XRFs) have, after the fast decline phase of their prompt emission, a temporal behaviour varying between two extremes. A large fraction of these AGs has a 'canonical' light curve which, after an initial shallow-decay 'plateau' phase, 'breaks smoothly' into a fast power-law decline. Very energetic GRBs, contrariwise, appear not to have a 'break', their AG declines like a power-law from the start of the observations. Breaks and 'missing breaks' are intimately related to the geometry and deceleration of the jets responsible for GRBs. In the frame of the 'cannonball' (CB) model of GRBs and XRFs, we analyze the cited extreme behaviours (canonical and pure power-law) and intermediate cases spanning the observed range of X-ray AG shapes. We show that the entire panoply of X-ray light-curve shapes --measured with Swift and other satellites-- are as anticipated, on very limpid grounds, by the CB model. We test the expected correlations between the AG's shape and the peak- and isotropic energies of the prompt radiation, strengthening a simple conclusion of the analysis of AG shapes: in energetic GRBs the break is not truly 'missing', it is hidden under the tail of the prompt emission, or it occurs too early to be recorded. We also verify that the spectral index of the unabsorbed AGs and the temporal index of their late power-law decline differ by half a unit, as predicted.