Evidence for chromatic x-ray light-curve breaks in swift grb afterglows and their theoretical implications
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6 pages
Published in:
- Mon.Not.Roy.Astron.Soc. 369 (2006) 2059
e-Print:
- astro-ph/0604105 [astro-ph]
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Abstract: (arXiv)
The power-law decay of the X-ray emission of GRB afterglows 050319, 050401, 050607, 050713A, 050802 and 050922C exhibits a steepening at about 1--4 hours 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 breaks 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 break arising from 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) were clearly required by pre-Swift afterglows.- radiation mechanisms: non-thermal
- shock waves
- ISM: jets and outflows
- gamma-rays: bursts
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