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New gamma-ray burst findings surprise astronomers

Paul Sutherland, Feature writer
Apr 30, 2014, 22:54 UTC

Sen—Astronomers have found that they must think again about what happens during the most powerful explosions in the Universe, known as gamma-ray bursts (GRBs).

Previous theoretical models of the behaviour of these monster blasts are mostly invalid, a new observational study of one event reveals.

A GRB, which makes a normal supernova look like a firecracker in comparison, is thought to occur when a really massive star, rotating very quickly, collapses into a black hole.

Due to its rapid spin, it produces jets of material that fire out from the top and bottom of its axis at a speed approaching that of light.

The first GRB was detected by American Vela satellites while watching for Soviet nuclear tests in 1967 at the height of the Cold War. It was quickly recognised that they were coming from deep space, but their source was a complete mystery.

Then after the Compton Gamma Ray Observatory was launched from the Space Shuttle Atlantis in 1991, it was seen that the short-lived bursts were happening randomly across the whole sky. This told astronomers that they must be occurring outside our own Milky Way galaxy.

In 1997 an Italian–Dutch X-ray astronomy satellite called BeppoSAX detected an afterglow from a GRB that showed the burst must have happened at a vast distance of more than six billion light-years and so must have been huge in size.

The new study, published today in the journal Nature, raises questions about the behaviour of such afterglows, which can last anything from days to weeks following the initial burst. It was led by scientists at the University of Leicester in the UK, adied by colleagues from the Niels Bohr Institute in Denmark.

They analysed a burst labelled GRB 121024A that occurred 10 billion light-years away, was detected on 24 October, 2012, by NASA's Swift satellite and which lasted just over a minute. Using the Very Large Telescope (VLT) in Chile, they checked to see what happened to the glow remaining after the burst, and in particular how the light was polarized.

Swift satellite

An artist's rendering of the Swift spacecraft that discovered the gamma-ray burst studied for the new paper. Image credit: NASA/Spectrum Astro

Dr Klaas Wiersema, of Leicester’s Department of Physics and Astronomy, said: “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.

“However, how this acceleration process actually works is very hard to study on Earth in laboratories, or using computer simulations. What we do, is study the polarized light of the afterglow using large optical telescopes, and special filters, that work much like the filters in Polaroid sunglasses.”

Dr Wiersema explained that light is a wave. When light is linearly polarized, it means that the wave vibrations lie in a plane, and when light is circularly polarized, it means that that this plane rotates on the sky.

He said: “Different theories for electron acceleration and light emission within the afterglow all predict different levels of linear polarization, but theories all agreed that there should be no circular polarization in visible light. This is where we come in: we decided to test this by carefully measuring both the linear and circular polarization of one afterglow, of GRB 121024A, detected by the Swift satellite.

“Using the VLT, we measured both the linear and circular polarization of an afterglow with high accuracy. Much to our surprise we clearly detected circular polarization, while theories predicted we should not see any at all.

A video about cosmic explosions by the Danish scientists. Credit: Niels Bohr Institute

“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. 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!”

Dr Wiersema added: “We are the first team to realize the importance of trying these technically difficult circular polarization measurements at visible wavelengths—most people simply assumed it wouldn’t be worthwhile doing as theory predicted levels too low to be detectable. The detection of far stronger circular polarization than expected makes it a particularly surprising result.

“We believe that this detection means that most of the current theories of how electrons get accelerated in afterglows need re-examining.”