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Jet breaks and energetics of swift gamma-ray burst X-ray afterglows

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posted on 2012-10-24, 09:21 authored by J. L. Racusin, D. N. Burrows, A. Falcone, E. W. Liang, B. B. Zhang, B. Zhang, T. Sakamoto, P. A. Evans, J. Osborne
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 (~12% of our sample), and reveal cases of potential jet breaks that do not appear plainly from the light curve alone (another ~30%), leading to insight into the missing jet break problem. Those X-ray 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 ~1 day, similar to what was found pre-Swift. On average, Swift GRBs have lower isotropic equivalent γ-ray energies, which in turn result in lower collimation corrected γ-ray energies than those of pre-Swift GRBs. Finally, we explore the implications for GRB jet geometry and energetics.


J.L.R., D.N.B., and A.F. gratefully acknowledge support for this work from NASA contract NAS5-00136. We acknowledge the use of public data from the Swift data archive. B.Z., B.B.Z, and E.-W.L. gratefully acknowledge support from NASA contracts NNG05GB67G, NNX08AN24G, NNX08AE57A, and a President’s Infrastructure Award from UNLV. E.-W.L. also acknowledges support from a National Natural Science Foundation of China grant 10873002, National Basic Research Program (“973” Program) of China (grant 2009CB824800), and the research foundation of Guangxi University.



Astrophysical Journal, 2009, 698 (1), pp. 43-74

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