JET BREAKS AND ENERGETICS OF<i>Swift</i>GAMMA-RAY BURST X-RAY AFTERGLOWS

Abstract

We present a systematic temporal and spectral study of all Swift-XRT\nobservations of GRB afterglows discovered between 2005 January and 2007\nDecember. After constructing and fitting all light curves and spectra to\npower-law models, we classify the components of each afterglow in terms of the\ncanonical X-ray afterglow and test them against the closure relations of the\nforward shock models for a variety of parameter combinations. The closure\nrelations are used to identify potential jet breaks with characteristics\nincluding the uniform jet model with and without lateral spreading and energy\ninjection, and a power-law structured jet model, all with a range of\nparameters. With this technique, we survey the X-ray afterglows with strong\nevidence for jet breaks (~12% of our sample), and reveal cases of potential jet\nbreaks that do not appear plainly from the light curve alone (another ~30%),\nleading to insight into the missing jet break problem. Those X-ray light curves\nthat do not show breaks or have breaks that are not consistent with one of the\njet models are explored to place limits on the times of unseen jet breaks. The\ndistribution of jet break times ranges from a few hours to a few weeks with a\nmedian of ~1 day, similar to what was found pre-Swift. On average Swift GRBs\nhave lower isotropic equivalent gamma-ray energies, which in turn results in\nlower collimation corrected gamma-ray energies than those of pre-Swift GRBs.\nFinally, we explore the implications for GRB jet geometry and energetics.\n