%0 Journal Article %A Panaitescu, A. %A Mészáros, P. %A Burrows, D. %A Nousek, J. %A Gehrels, N. %A O'Brien, P. %A Willingale, R %D 2012 %T Evidence for chromatic X-ray light-curve breaks in Swift gamma-ray burst afterglows and their theoretical implications %U https://figshare.le.ac.uk/articles/journal_contribution/Evidence_for_chromatic_X-ray_light-curve_breaks_in_Swift_gamma-ray_burst_afterglows_and_their_theoretical_implications/10113827 %2 https://figshare.le.ac.uk/ndownloader/files/18229766 %K IR content %X The power-law decay of the X-ray emission of gamma-ray burst (GRB) afterglows 050319, 050401, 050607, 050713A, 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. %I University of Leicester