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The widely existing shallow decay phase of the X-ray afterglows of gamma-raybursts (GRBs) is generally accepted to be due to long-lasting energy injection.The outflows carrying the injecting energy, based on the component that isdominative in energy, fall into two possible types: baryon-dominated andlepton-dominated ones. The former type of outflow could be ejecta that isejected during the prompt phase of a GRB and consists of a series of baryonicshells with a distribution of Lorentz factors, and the latter type could be anelectron-positron-pair wind that is driven by the post-burst central engine. Wehere provide a unified description for the dynamics of fireballs based on thesetwo types of energy injection, and calculate the corresponding high-energyphoton emission by considering synchrotron radiation and inverse Comptonscattering (including synchrotron self-Compton and combined inverse-Compton) ofelectrons. We find that, in the two energy-injection models, there is a plateau(even a hump) in high-energy light curves during the X-ray shallow decay phase.In particular, a considerable fraction of the injecting energy in thelepton-dominated model can be shared by the long-lasting reverse shock since itis relativistic. Furthermore, almost all of the energy of the reverse shock iscarried by leptons, and thus the inverse-Compton emission is enhanceddramatically. Therefore, this model predicts more significant high-energyafterglow emission than the baryon-dominated model. We argue that theseobservational signatures would be used to discriminate between differentenergy-injection models in the upcoming {\em Gamma-ray Large Area SpaceTelescope} (GLAST) era.

Observational Signatures of High-Energy Emission during the Shallow Decay Phase of GRB X-Ray Afterglows