  The two-component emission model to explain the plateau phase of the X-ray
afterglows of gamma-ray bursts (GRBs) is proposed. One component, which is
responsible for the plateau and subsequent normal decay phase of the X-ray
afterglow, is the prior emission via outflow ejected from the central engine
before the main burst. The other is the main outflow, which causes the prompt
GRB emission and the initial steep decay phase of the X-ray afterglow. In this
model, the transition from the plateau to the subsequent normal decay phase is
an artifact of the choice of the zero of time. For events with distinct plateau
phase, the central engine is active 10^3-10^4 sec before the launch of the main
outflow. According to this model, a prior emission in the X-ray and/or optical
bands 10^3-10^4 sec before the prompt GRB emission is possibly seen, which will
be tested by near-future instruments such as Monitor of All-sky X-ray Image
(MAXI), WIDe-field telescope for GRB Early Timing (WIDGET), and so on.
