The site and mechanism of gamma-ray burst (GRB) prompt emission is stillunknown. Although internal shocks have been widely discussed as the emissionsite of GRBs, evidence supporting other emission sites have been also suggestedrecently, including the closer-in photosphere where the fireball becomestransparent and further-out radii near the fireball deceleration radius wheremagnetic dissipation may be important. With the successful operation of theGLAST experiment, prompt high energy emission spectra from many GRBs would bedetected in the near future. We suggest that the cut-off energy of the promptemission spectrum from a GRB depends on both the fireball bulk Lorentz factorand the unknown emission radius from the central engine. If the bulk Lorentzfactor could be independently measured (e.g. from early afterglowobservations), the observed spectral cutoff energy can be used to diagnose theemission site of gamma-rays. This would provide valuable information tounderstand the physical origin of the GRB promp emission.
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