Shockwave: New Findings On Gamma Ray Bursts Rewrite Old Theories

By Shweta Iyer on April 30, 2014 4:38 PM EDT

gamma ray burst
Measurements of polarized light in the afterglow of GRB 120308A by the Liverpool Telescope and its RINGO2 instrument indicate the presence of a large-scale stable magnetic field linked with a young black hole, as shown in this illustration. (Photo: NASA's Goddard Space Flig)

Gamma-Ray Bursts (GRB) are the brightest electromagnetic radiations known to occur in the universe. Several theories have been proposed on how electrons present in the GRB afterglow get accelerated. But now an international team of scientists led by the University of Leicester has discovered for the first time that GRB's behave differently than previously thought. The findings have been published in the journal Nature, according to a press release Wednesday. 

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Dr Klaas Wiersema, of the University of Leicester's Department of Physics and Astronomy explains, "When a suitable GRB is detected by a satellite, I get a text message on my phone, and then I have to very quickly tell the observatory in Chile exactly which observations I want them to take, and how."

"About once per day, a short, very bright flash of gamma-rays (the most energetic form of light) is detected by satellites. These flashes are called gamma-ray bursts (GRBs), and take place in galaxies far away, when a massive star collapses at the end of its life."

"These GRBs are followed by a so-called "afterglow", slowly fading emission that can be seen at all wavelengths (including visible light), 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."

"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 polarised light of the afterglow using large optical telescopes, and special filters, that work much like the filters in Polaroid sunglasses."

Dr. Wiersema goes on to explain polarised light. Light is a wave and when all the light waves vibrate the same way they are polarized. In other words when wave vibrations oscillate in one plane they are linearly polarized and when the plane of oscillation rotates on the sky it is circularly polarized.

He added: "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. This is where we come in: we decided to test this by carefully measuring both the linear and circular polarisation of one afterglow, of GRB 121024A, detected by the Swift satellite.

"Using the Very Large Telescope (VLT) in Chile, we measured both the linear and circular polarisation of an afterglow with high accuracy. Much to our surprise we clearly detected circular polarisation, while theories predicted we should not see any at all. 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 observes like me are in this game!

We are the first team to realise the importance of trying these technically difficult circular polarisation 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 polarisation 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."

Taking high precision measurements of an afterglow is quite difficult technically, so Dr Wiersema and his team consider their research as path-breaking and are working towards more such observations.

He says, "Extreme shocks like the ones in GRB afterglows are great natural laboratories to push our understanding of physics beyond the ranges that can be explored in laboratories."

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