Evidence for chromatic X-ray light-curve breaks in Swift gamma-ray burst afterglows and their theoretical implications

A. Panaitescu, P. Mészáros, D. Burrows, J. Nousek, N. Gehrels, P. O'Brien, R. Willingale

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The power-law decay of the X-ray emission of gamma-ray burst (GRB) afterglows 050319,050401, 050607,050713 A, 050802 and 050922C exhibits a steepening at about 1-4 h after the burst which, surprisingly, is not accompanied by a break in the optical emission. If it is assumed that both the optical and X-ray afterglows arise from the same outflow then, in the framework of the standard forward shock model, the chromaticity of the X-ray light-curve breaks indicates that they do not arise solely from a mechanism related to the outflow dynamics (e.g. energy injection) or the angular distribution of the blast-wave kinetic energy (structured outflows or jets). The lack of a spectral evolution accompanying the X-ray light-curve break shows that these breaks do not arise from the passage of a spectral break (e.g. the cooling frequency) either. Under these circumstances, the decoupling of the X-ray and optical decays requires that the microphysical parameters for the electron and magnetic energies in the forward shock evolve in time, whether the X-ray afterglow is synchrotron or inverse-Compton emission. For a steady evolution of these parameters with the Lorentz factor of the forward shock and an X-ray light curve arising cessation of energy injection into the blast wave, the optical and X-ray properties of the above six Swift afterglows require a circumburst medium with a r-2 radial stratification, as expected for a massive star origin for long GRBs. Alternatively, the chromatic X-ray light-curve breaks may indicate that the optical and X-ray emissions arise from different outflows. Neither feature (evolution of microphysical parameters or the different origin of the optical and X-ray emissions) was clearly required by pre-Swift afterglows.

Original languageEnglish (US)
Pages (from-to)2059-2064
Number of pages6
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - Jul 2006

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


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