Gamma-ray burst challenges particle acceleration theories

For the first time circular polarization has been observed in the afterglow of a gamma-ray burst (GRB) -- the explosive death of a massive star.

The light from the afterglow is 10,000 times more strongly circularly polarized than expected.

The current theoretical models that describe particle acceleration in a GRB are unable to explain this surprising observation.

GRBs are powerful, transient explosions in the distant universe that occur when a massive star explodes and a black hole is formed. The GRB lasts a few minutes, but the afterglow remains observable in visible light for a few days. The afterglow is formed when the jets of matter that are launched from the black hole collide with the surrounding matter, and create a shockwave which involves particle acceleration.

From the observation of the afterglow of GRB 121024A it turns out that the light is not only linearly polarized-it propagates in one plane-but also circularly polarized-it propagates around an axis and follows a path like a corkscrew. This circular polarization has been measured for the first time for a GRB and is 6 to 7 times weaker than the linear polarization, but much stronger than predicted.

GRB 121024A was detected with the Swift satellite on 24 October 2012. The afterglow was observed with ESO's Very Large Telescope (VLT) in Chile during the following two days. Although the afterglow was not exceptionally bright and its linear polarization was of average strength, the team was able to measure circular polarization against all expectations.

The study has been published online in the journal Nature.