A Late‐Time Flattening of Afterglow Light Curves

We present a sample of radio afterglow light curves with measured decayslopes which show evidence for a flattening at late times compared to opticaland X-ray decay indices. The simplest origin for this behavior is that thechange in slope is due to a jet-like outflow making a transition tosub-relativistic expansion. This can explain the late-time radio light curvesfor many but not all of the bursts in the sample. We investigate severalpossible modifications to the standard fireball model which can flattenlate-time light curves. Changes to the shock microphysics which govern particleacceleration, or energy injection to the shock (either radially or azimuthally)can reproduce the observed behavior. Distinguishing between these differentpossibilities will require simultaneous optical/radio monitoring of afterglowsat late times.Comment: ApJ, submitte