Circularly polarized light from a gamma-ray burst

Gamma-ray bursts (GRBs) are the most violent explosions in the universe. Most of them are likely powered by jets of relativistic particles that are launched from dying stars as they collapse into a compact object such as a black hole. They are named for their initial blast of high-energy radiation, but GRBs can glow for days, emitting radiation across the electromagnetic spectrum. Studies of the linear polarization of GRB light provide information about the magnetic field in the jet and help clarify jet geometry. Now, a collaboration led by Klaas Wiersema of the University of Leicester in the UK reports having observed circularly polarized optical light in the radiation of a GRB detected on 24 October 2012. The strength of the signal, measured a few hours subsequent to the initial burst, in the afterglow phase, was orders of magnitude above what simple models predict. Scattering from dust can convert linearly...