Phases of Swift X-ray Afterglows

Abstract

The X-ray afterglows observed by Swift exhibit rich light-curves, with four phases of different decay rate. The temporal and spectral properties for a set of 47 bursts are used to identify the mechanisms which can explain these four phases. The early, fast-decaying phase can be attributed to the same mechanism which generated the burst emission (internal shocks in a relativistic outflow), while the following phases of slower decay can be identified with synchrotron emission from the forward shock sweeping the circumburst medium. Most likely, the phase of slowest decay is due to a continuous energy injection in the forward shock. That the optical power-law decay continues unabated after the end of energy injection requires an ambient medium with a wind-like density structure (n propto r^{-2}) and forward shock microphysical parameters that change with the shock's Lorentz factor. A later break of the X-ray light-curve can be attributed to a collimated outflow whose boundary becomes visible to the observer (a jet) but the optical and X-ray decays are not always consistent with the standard jet model expectations.