JET BREAKS AND ENERGETICS OF Swift GAMMA-RAY BURST X-RAY AFTERGLOWS

We present a systematic temporal and spectral study of all Swift-X-ray Telescope observations of gamma-ray burst (GRB) afterglows discovered between 2005 January and 2007 December. After constructing and fitting all light curves and spectra to power-law models, we classify the components of each afterglow in terms of the canonical X-ray afterglow and test them against the closure relations of the forward shock models for a variety of parameter combinations. The closure relations are used to identify potential jet breaks with characteristics including the uniform jet model with and without lateral spreading and energy injection, and a power-law structured jet model, all with a range of parameters. With this technique, we survey the X-ray afterglows with strong evidence for jet breaks (similar to 12% of our sample), and reveal cases of potential jet breaks that do not appear plainly from the light curve alone (another similar to 30%), leading to insight into the missing jet break problem. Those Xray light curves that do not show breaks or have breaks that are not consistent with one of the jet models are explored to place limits on the times of unseen jet breaks. The distribution of jet break times ranges from a few hours to a few weeks with a median of similar to 1 day, similar to what was found pre-Swift. On average, Swift GRBs have lower isotropic equivalent gamma-ray energies, which in turn result in lower collimation corrected gamma-ray energies than those of pre-Swift GRBs. Finally, we explore the implications for GRB jet geometry and energetics.