The Interpretation and Implication of the Afterglow of GRB 060218

Abstract

The nearby GRB 060216/SN 2006aj was an extremely long, weak and very soft GRB. It was peculiar in many aspects. We show here that the X-ray, ultraviolet/optical and radio afterglow of GRB 060218 have to be attributed to different physical processes arising from different emission regions. From the several components in this burst's afterglow only the radio afterglow can be interpreted in terms of the common external shock model. We infer from the radio that the blast wave's kinetic energy was $\sim 10^{50}$ erg and the circumburst matter had a constant rather than a wind profile. The lack of a "jet break" up to 22 days implies that the outflow was wide $θ_j >1$. Even though the late X-ray afterglow decays normally it cannot result from an external shock because of its very steep spectrum. Furthermore, the implied kinetic energy would have produced far too much radio. We suggest that this X-ray afterglow could be attributed to a continued activity of the central engine that within the collapsar scenario could arise from fall-back accretion. "Central engine afterglow" may be common in under-luminous GRBs where the kinetic energy of the blast wave is small and the external shock does not dominate over this component. Such under-luminous GRBs might be very common but they are rarely recorded because they can be detected only from short distances.