  Gamma-ray bursts (GRBs) are most probably powered by collimated relativistic
outflows (jets) from accreting black holes at cosmological distances. Bright
afterglows are produced when the outflow collides with the ambient medium.
Afterglow polarization directly probes the magnetic properties of the jet, when
measured minutes after the burst, and the geometric properties of the jet and
the ambient medium when measured hours to days after the burst. High values of
optical polarization detected minutes after burst in GRB 120308A indicate the
presence of large-scale ordered magnetic fields originating from the central
engine (the power source of the GRB). Theoretical models predict low degrees of
linear polarization and negligable circular polarization at late times, when
the energy in the original ejecta is quickly transferred to the ambient medium
and propagates farther into the medium as a blastwave. Here we report the
detection of circularly polarized optical light in the afterglow of GRB
121024A, measured 0.15 days after the burst. We show that the circular
polarization is intrinsic to the afterglow and unlikely to be produced by dust
scattering or plasma propagation effects. A possible explanation is to invoke
anisotropic (rather than the commonly assumed isotropic) electron pitch angle
distributions, and we suggest that new models are required to produce the
complex microphysics of realistic shocks in relativistic jets.
