Circular polarization in the optical afterglow of GRB 121024A

Understand

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).