Relativistically expanding sources of X-rays and gamma-rays cover an enormous range of (central) compactness and Lorentz factor. The underlying physics is discussed, with an emphasis on how the dominant dissipative mode and the emergent spectrum depend on these parameters. Photons advected outward from high optical depth are a potentially important source of Compton seeds. Their characteristic energy is bounded below by ~1 MeV in pair-loaded outflows of relatively low compactness, and remains near ~1 MeV at very high compactness and low matter loading. This is compared with the characteristic energy of O(1) MeV observed in the rest frame spectra of many sources, including gamma-ray bursts, OSSE jet sources, MeV Blazars, and the intense initial 0.1 s pulse of the March 5 event. Additional topics discussed include the feedback of pair creation on electron heating and the formation of non-thermal spectra, their effectiveness at shielding the dissipative zone from ambient photons, direct Compton damping of irregularities in the outflow, the relative importance of various soft photon sources, and the softening of the emergent spectrum that results from heavy matter loading. The implications of this work for X-ray and optical afterglow from GRB’s are briefly considered. Direct synchrotron emission behind the forward shock is inhibited by the extremely low energy density of the ambient magnetic field. Mildly relativistic ejecta off axis from the main gamma-ray emitting cone become optically thin to scattering on a timescale of ~1 day (E/10^{52} erg)^{1/2}, and can be a direct source of afterglow radiation.
Date added: Thu, 10 Oct 13