Sustaining large-scale magnetic fields in gamma-ray burst afterglows

Relativistic shocks play important roles in accelerating non-thermal particles and in producing the long-lasting synchrotron afterglow emission of gamma-ray bursts (GRBs). Current models of GRB afterglow shocks fail to produce strong magnetic fields on the spatial scales necessary to explain the observed synchrotron emission. In the early stages of GRB shocks, prompt MeV photons can pair-produce and load the pre-shock medium with electron-positron pairs. Using first-principles plasma simulations, we show how the continuous injection of electron-positron pairs ahead of the shock affects the growth of magnetic fields. As we increase the duration of pair injection, the magnetic fields grow to progressively larger scales and thus survive for longer times, mitigating the tension between theoretical models and observations of GRB afterglows. These results provide us with a better understanding of the properties of the self-generated fields in GRB shocks and of the resulting synchrotron emission in GRB afterglows.