Linear Polarization in Gamma-Ray Bursts: The Case for an Ordered Magnetic

Linear polarization at the level of ∼1%–3 % has by now been measured in several gamma-ray burst afterglows. Whereas the degree of polarization, P, was found to vary in some sources, the position angle, , was roughlyvp constant in all cases. Until now, the polarization has been commonly attributed to synchrotron radiation from a jet with a tangled magnetic field that is viewed somewhat off-axis. However, this model predicts either a peak in P or a 90 change in around the “jet break ” time in the light curve, for which there has so far been novp observational confirmation. We propose an alternative interpretation, wherein the polarization is attributed, at least in part, to a large-scale, ordered magnetic field in the ambient medium. The ordered component may dominate the polarization even if the total emissivity is dominated by a tangled field generated by postshock turbulence. In this picture, is roughly constant because of the uniformity of the field, whereas P varies as a result ofvp changes in the ratio of the ordered-to-random mean-squared field amplitudes. We point out that variable afterglow light curves should be accompanied by a variable polarization. The radiation from the original ejecta, which includes the prompt g-ray emission and the emission from the reverse shock (the “optical flash ” and “radio flare”), could potentially exhibit a high degree of polarization (up to ∼60%) induced by an ordered transverse magnetic field advected from the central source