Jitter Radiation as a Possible Mechanism for Gamma-Ray Burst Afterglows: Spectra and Light Curves

The standard model of gamma-ray burst afterglows assumes that the radiation observed as a delayed emission is of synchrotron origin, which requires the shock magnetic field to be relatively homogeneous on small scales. An alternative mechanism—jitter radiation, which traditionally has been applied to the prompt emission—substitutes for synchrotron when the magnetic field is tangled on a microscopic scale. Such are the fields produced at relativistic shocks by the Weibel instability. Here we explore the possibility that small-scale fields populate afterglow shocks. We derive the spectrum of jitter radiation under the afterglow conditions. We also derive the afterglow light curves for the interstellar medium and wind profiles of the ambient density. Jitter self-absorption is calculated here for the first time. We find that jitter radiation can produce afterglows similar to synchrotron-generated ones, but with some important differences. We compare the predictions of the two emission mechanisms. With future observational data, one may be able to discriminate between the synchrotron and jitter afterglow light curves, and, hence, between the small-scale versus large-scale magnetic field models in afterglow shocks.