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

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.

To be published in Ap. J

References in corpus (11)

• An online repository of Swift/XRT light curves of GRBs
• The Hubble Diagram to Redshift >6 from 69 Gamma-Ray Bursts
• Swift Identification of Dark Gamma-Ray Bursts
• The Remarkable Afterglow of GRB 061007: Implications for Optical Flashes and GRB Fireballs
• GRB Energetics in the Swift Era
• Temporal Profiles and Spectral Lags of XRF 060218
• Swift XRT Observations of the Afterglow of XRF 050416A
• Observations of the Prompt Gamma-Ray Emission of GRB 070125
• The rapid decline of the prompt emission in Gamma-Ray Bursts
• On the X-Ray emission of Gamma Ray Bursts
• An introduction to Cosmic Rays and Gamma-Ray Bursts, and to their simple understanding

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• Is there anything special about GRB 080319B?
• Interpreting the X-ray Flash XRF 060218 and its associated supernova