2004ApJ...600..828F

We present a sample of radio afterglow light curves with measured decay slopes that show evidence of a flattening at late times compared to optical and X-ray decay indices. The simplest origin for this behavior is that the change in slope is due to a jetlike outflow making a transition to subrelativistic expansion. This can explain the late-time radio light curves for some but not all of the bursts in the sample. We investigate several possible modifications to the standard fireball model, which can flatten late-time light curves. Changes to the shock microphysics, which govern particle acceleration, or energy injection to the shock (either radially or azimuthally) can reproduce the observed behavior. Distinguishing between these different possibilities will require simultaneous optical/radio monitoring of afterglows at late times.