High-energy spectral components in gamma-ray burst afterglows

We investigate two high-energy radiation mechanisms, the proton-synchrotron and the electron inverse Compton emission, and explore their possible signatures in the broadband spectra and in the keV-GeV light curves of gamma-ray burst afterglows. We develop a simple analytical approach, also allowing for the effects of photon-photon pair production, and explore the conditions under which one or the other of these components dominates. We identify three parameter-space regions in which different spectral components dominate: (1) a region where the proton-synchrotron and other hadron-related emission components dominate, which is small; (2) a region in which the electron inverse Compton component dominates, which is substantial; and (3) a third substantial region in which electron-synchrotron emission dominates. We discuss the prospects and astrophysical implications of directly detecting the inverse Compton and the proton high-energy components in various bands, in particular, in the GeV band, with future missions such as the Gamma-Ray Large Area Space Telescope (GLAST) and in the X-ray band with Chandra. We find that regime II parameter space is the most favorable regime for high-energy emission. The inverse Compton component is detectable by GLAST within hours for bursts at typical cosmological distances and by Chandra within days if the ambient density is high.