Open AccessEvidence for chromatic X‐ray light‐curve breaks in Swift gamma‐ray burst afterglows and their theoretical implications
Author(s) -
Panaitescu A.,
Mészáros P.,
Burrows D.,
Nousek J.,
Gehrels N.,
O'Brien P.,
Willingale R.
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10453.x
Subject(s) - physics , light curve , gamma ray burst , afterglow , astrophysics , lorentz factor , spectral slope , shock wave , astronomy , spectral line , lorentz transformation , classical mechanics , thermodynamics
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.