For the first time circular polarisation 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 polarised than expected. The current theoretical models that describe particle acceleration in a GRB are unable to explain this surprising observation. The study, by a team that includes astronomers from the University of Amsterdam, has been published online in Nature on 30 April 2014.

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 polarised - it propagates in one plane - but also circularly polarised - it propagates around an axis and follows a path like a corkscrew. This circular polarisation has been measured for the first time for a GRB and is 6 to 7 times weaker than the linear polarisation, but much stronger than predicted.

"This finding is a huge surprise to us. According to theoretical models the circular polarisation is too weak to be measured, but apparently particle acceleration and the role of the structure of the magnetic field in jets is not fully understood", says Alexander van der Horst from the University of Amsterdam (UvA). First author Klaas Wiersema adds: "We believe that this detection means that most of the current theories of how particles get accelerated in afterglows need re-examining".

GBR 121024A was detected with the SWIFT-satellite on 24 October 2010. 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 polarisation was of average strength, the team was able to measure circular polarisation against all expectations.

UvA astronomer Michiel Min: "It is a challenge to measure circular polarisation in the afterglow of GRB's because the right telescope and instrument are required. We have been very lucky this time. In this case we were able to point the VLT and its FORS2 instrument at the source directly."

Jets are a common phenomenon in the universe. They are not only present in GRBs, but also in sources like Active Galactic Nuclei. "The formation of jets in similar objects remains a mystery. These unique observations of circular polarisation help us to understand them better", says Ralph Wijers (UvA).

Nature paper