Gamma-ray Burst
May 1, 2014

Circular Polarization Observed In Gamma-Ray Burst Afterglow

redOrbit Staff & Wire Reports - Your Universe Online

An international team of astronomers has for the first time measured circular polarization in the afterglow of gamma-ray bursts, the powerful flashes of radiation originating from massive dying stars as they collapse into black holes.

A massive star explodes as a supernova when it dies, and its core collapses into a black hole, scientists from the Niels Bohr Institute explained in a statement Wednesday. In rare instances, a jet is formed along the black hole’s rotation axis, and processes that take place in that jet emit gamma radiation in the form of gamma-ray bursts.

Typically, those bursts only last a few minutes, but after a shockwave collides with material surrounding the dying star, it forms what is known as an afterglow. The afterglow can be observed for several days following the actual burst, and experts have previously developed theoretical models of the process. However, the authors of a paper published online Wednesday in the journal Nature have discovered that this afterglow behaves differently than anticipated.

According to Dr. Peter Curran of the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) and his colleagues, observations of Gamma-ray Burst 121024A made using the Very Large Telescope (VLT) in Chile revealed that its light was “circularly polarized,” meaning that the waves are moving together in a uniform, corkscrew-style spiral motion as they travel.

“Most light in the natural world is unpolarized, the waves are bouncing around at random,” explained Dr. Curran in a separate statement. “But the light from this Gamma-ray Burst looked like it was part of a 3D movie – it was about 1000 times more polarized than we expected. This means that the assumptions we've been making about Gamma-ray Bursts need to be completely reconsidered – assumptions of how electrons are accelerated to the incredible speeds we observe.”

Gamma-ray bursts are the brightest objects in the entire universe, sending out as much energy in a fraction of a second as our sun will give off during its entire lifespan, the study authors explained. Dr. Curran compared the bursts, which travel at 99.995 percent the speed of light, to amped-up versions of the Large Hadron Collider (the largest and most powerful particle accelerator in the world).

“Our results show that Gamma-ray Bursts are far more complex than we thought,” he said, noting that this was the first time astronomers have found circular polarization in the light from a gamma-ray burst. “We can use them to study microscopic electrons and how they behave in extreme environments, at a great distance – in this case, 18,500 million light years away, at a time when the Universe was just a fraction of its current age.”

Johan Fynbo and Jens Hjorth, professors at the Dark Cosmology Centre at the Niels Bohr Institute at the University of Copenhagen who were also involved in the research, explained that the gamma-ray burst actually occurred approximately 10 billion years ago, but the signal only reached Earth for the first time on October 24, 2012.

“The gamma-ray burst itself lasted just over a minute,” they said. “The subsequent afterglow was relatively bright, which allowed us to study it using more advanced methods than previously possible. Specifically, it was possible to measure the degree of linear and circular polarization during the first two days after the burst.”

“The theory for the distribution of radiation in the afterglow predicts a particular distribution, where circularly polarized light only representing a very minimal share, but we discovered that the reality is different.” Fynbo and Hjorth added, explaining that there was 10,000 time more circularly polarized light than expected. Scientists from the UK, Italy, Japan, China, Germany, Australia, Spain, Slovenia, and United Arab Emirates were also involved in the research.