Time-reversed Gamma-Ray Burst Light-curve Characteristics as Transitions between Subluminal and Superluminal Motion

We introduce a simple model to explain the time-reversed and stretched residuals in gamma-ray burst (GRB) pulse light curves. In this model an impactor wave in an expanding GRB jet accelerates from subluminal to superluminal velocities, or decelerates from superluminal to subluminal velocities. The impactor wave interacts with the surrounding medium to produce Cerenkov and/or other collisional radiation when traveling faster than the speed of light in this medium, and other mechanisms (such as thermalized Compton or synchrotron shock radiation) when traveling slower than the speed of light. These transitions create both a time-forward and a time-reversed set of light-curve features through the process of relativistic image doubling. The model can account for a variety of unexplained yet observed GRB pulse behaviors, including the amount of stretching observed in time-reversed GRB pulse residuals and the relationship between stretching factor and pulse asymmetry. The model is applicable to all GRB classes since similar pulse behaviors are observed in long/intermediate GRBs, short GRBs, and X-ray flares. The free model parameters are the impactor's Lorentz factor when moving subluminally, its Lorentz factor when moving superluminally, and the speed of light in the impacted medium.