  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
Cherenkov 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 (RID).
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.
