Phases of Swift X-ray Afterglows

TL;DR

Swift X-ray afterglows display four distinct phases with varying decay rates, explained by different physical mechanisms including internal shocks and synchrotron emission, with some phases influenced by energy injection and jet geometry.

Contribution

This paper identifies and explains the four phases of Swift X-ray afterglows using a comprehensive analysis of 47 bursts, highlighting the roles of internal shocks, synchrotron emission, energy injection, and jet effects.

Findings

Early phase due to internal shocks

Slower decay phases from synchrotron emission

Energy injection influences slowest decay

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.