  Since its launch in 2004, the Swift satellite has monitored the X-ray
afterglows of several hundred Gamma-Ray Bursts, and revealed that their X-ray
light-curves are more complex than previously thought, exhibiting up to three
power-law segments. Energy injection into the relativistic blast-wave
energizing the burst ambient medium has been proposed most often to be the
reason for the X-ray afterglow complexity. We examine 117 light-curve breaks of
98 Swift X-ray afterglows, selected for their high-quality monitoring and
well-constrained flux decay rates. Thirty percent of afterglows have a break
that can be an adiabatic jet-break, in the sense that there is one variant of
the forward-shock emission from a collimated outflow model that can account for
both the pre- and post-break flux power-law decay indices, given the measured
X-ray spectral slope. If allowance is made for a steady energy injection into
the forward-shock, then another 56 percent of X-ray afterglows have a
light-curve break that can be explained with a jet-break. The remaining 12
percent that are not jet-breaks, as well as the existence of two breaks in 19
afterglows (out of which only one can be a jet-break), suggest that some X-ray
breaks arise from a sudden change in the rate at which energy is added to the
blast-wave, and it may well be that a larger fraction of X-ray light-curve
breaks are generated by that mechanism. To test the above two mechanisms for
afterglow light-curve breaks, we derive comprehensive analytical results for
the dynamics of outflows undergoing energy injection and for their
light-curves, including closure relations for inverse-Compton afterglows and
for the emission from spreading jets interacting with an wind-like ambient
medium.
