Probing Electron Acceleration in Gamma-Ray Burst Afterglows

Many high-energy astrophysical sources accelerate electrons to relativistic velocities, resulting in broadband emission from synchrotron and other radiation processes. Gamma-ray burst afterglow observations provide a unique probe of electron acceleration due to the relativistic nature of their collimated outflows. The peaks in gamma-ray burst radio light curves and spectral energy distributions pin down the characteristic synchrotron frequency related to the minimum Lorentz factor of the accelerated electrons, Γ_m. We present a method that constrains the fraction of shock energy that resides in electrons, ε_e, and the fraction of electrons that is shocked into a power-law energy distribution, ξ. Based on a large sample of radio afterglows, we show narrow distributions for ε_e and Γ_m, largely independent of other physical parameters describing the micro- and macrophysics of these gamma-ray burst jets and their environment. We also put constraints on ξ, which is almost impossible to do with broadband modeling due to its degeneracy with other physical parameters. These results have implications for multi-wavelength modeling of gamma-ray burst afterglows and the derived physical parameters, and for simulations of electron acceleration in relativistic shocks, which is relevant for a large variety of high-energy astrophysical sources.