New understanding of gamma-ray bursts

New observations suggest that gamma-ray bursts behave differently than previously thought. Theories predicted that light from the afterglow of a gamma-ray burst (GRB) would only be polarised linearly and not circularly. However, a team of international researcher have clearly detected circular polarisation in the afterglow of GRB 121024A.The finding could change theoretical understanding of the afterglows of GRB. GRBs are short, bright flashes of gamma-rays which occur when a massive star collapses at the end of its life. They are followed by a so-called afterglow, a slowly fading emission which can be seen at all wavelengths for a few days to weeks. “We know that the afterglow emission is formed by a shockwave, moving at very high velocities, in which electrons are being accelerated to tremendous energies. These fast moving electrons then produce the afterglow light that we detect,” said Dr Klass Wiersema from the University of Leicester was part of the team. “However, how this acceleration process actually works is very hard to study on Earth in laboratories or using computer simulations.” Instead, researchers study the polarised light of the afterglow using large optical telescopes and special filters. Light can be polarised linearly – meaning wave vibrations line in a plane – or circularly, meaning the plane rotates in the sky. “Different theories for electron acceleration and light emission within the afterglow all predict different levels of linear polarisation, but theories all agreed that there should be no circular polarisation in visible light,” said Wiersema. “Much to our surprise we clear detected circular polarisation of an afterglow with high accuracy. We believe that the most likely explanation is that the exact way in which electrons are accelerated within the afterglow shockwave is different from what we always thought.” The paper, published in Nature, states that the circular polarisation is intrinsic to the afterglow and unlikely to be produced by dust scattering or plasma propagation effects. “It is a very nice example of observations ruling out most of the existing theoretical predictions – exactly why observers like me are in this game,” Wiersema added. Circular polarization in the optical afterglow of GRB 121024A