Electron Acceleration and Efficiency in Nonthermal Gamma-Ray Sources

In energetic nonthermal sources such as gamma-ray bursts, AGN or galactic jetsources, etc., one expects both relativistic and transrelativistic shocksacompanied by violent motions of moderately relativistic plasma. We presentgeneral considerations indicating that these sites are electron and positronaccelerators leading to a modified power law spectrum. The electron (or$e^\pm$) energy index is very hard, $\propto \gamma^{-1}$ or flatter up to acomoving frame break energy $\gamma_\ast$, and becomes steeper above that. Inthe example of gamma-ray bursts the Lorentz factor reaches $\gamma_\ast\sim10^3$ for $e^{\pm}$ accelerated by the internal shock ensemble onsubhydrodynamical time scales. For pairs accelerated on hydrodynamicaltimescales in the external shocks similarly hard spectra are obtained, and thebreak Lorentz factor can be as high as $\gamma_\star \siml 10^5$. Radiationfrom the nonthermal electrons produces photon spectra with shape andcharacteristic energies in qualitative agreement with observed genericgamma-ray burst and blazar spectra. The scenario described here provides aplausible way to solve one of the crucial problems of nonthermal high energysources, namely the efficient transfer of energy from the proton flow to anapropriate nonthermal lepton component.Comment: Ap.J. (Letters) in press, uuencoded latex file (uses AAS macro aaspp4), 10 page