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

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 in the CB model. We test the expected correlations between the AG’s shape and the peakand 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. [email protected] Physics Department and Space Research Institute, Technion, Haifa 32000, Israel [email protected]; [email protected] Physics Department and Space Research Institute, Technion, Haifa 32000, Israel Theory Unit, CERN, 1211 Geneva 23, Switzerland [email protected] Theory Unit, CERN,1211 Geneva 23, Switzerland and Physics Department, Boston University, USA