Universe's biggest explosions captured in unprecedented detail

An international team of scientists, including from the University of Bath, have captured the most complete picture yet of the most powerful type of explosion in the universe – Gamma Ray Bursts - unravelling the long-standing mystery of what powers them.

Short-lived gamma ray bursts (GRBs) are intense flashes of high-energy light detected by space-based telescopes orbiting above the Earth’s atmosphere. Their bright flashes are thought to represent the birth of a distant black hole, formed in the death throes of a massive star as it explodes as a supernova. But such prodigious energies have been hard to explain with standard explosion theories.

Instead, scientists have long thought that the energy of magnetic fields could provide the answer. However proving this is a major technological challenge as these explosions lie many millions of light years from Earth and are gone in seconds or minutes, never to repeat.

An artist's impression of a Gamma Ray Burst. Credit ESO/A. Roquette

Until recently, traditional ground-based telescopes that require human intervention to make astronomical measurements have proven too slow to capture the fast-fading light from these transient explosions.

By an incredible one-in-10,000 chance the international team from the University of Bath, NASA, University of Maryland and others around the world, detected light from one of these extremely rare bursts beginning as a dying star collapsed into a black hole.

Using novel, autonomous robotic telescopes, the team were able not only to measure how the light was produced by the material ejected into space during the explosion but most importantly, they measured a special property of the light that probes magnetic fields – it’s polarisation.

The results are published in Nature.

Professor Carole Mundell, Head of Physics at the University of Bath, world-leading expert in rapid autonomous, robotic technology for the measurement of cosmic magnetic fields and co-author on the new paper said: “The origin and nature of magnetic fields is one of largest unsolved problems in modern astrophysics. GRBs are natural laboratories for extreme physics and the technique we use allows us to probe magnetic fields directly.

“Although very distant, this burst was extremely bright and we were excited when we realised our super-fast robotic telescopes had captured the early time light and that we might have a chance of probing the magnetic fields very close to the black hole itself by measuring the polarisation properties of the light and following how this changed in time, as the explosion developed.”

The group’s measurements provide the first answers to some long-standing questions about how GRBS evolve during a star’s collapse. Their data suggest strong magnetic fields form close to the new black hole and drive energy and material outwards in a tightly focused beam.

Professor Mundell added: ‘We’ve shown previously using slower autonomous robotic telescopes that magnetic fields must be important and help to guide the material outwards at high speed but, until now, we were never fast enough to capture bright visible light at the same time as the high energy gamma rays produced during the explosion itself.

Professor Carole Mundell

“There is intense debate about the nature of these high speed flows - how material can be accelerated to such high speeds, what physical mechanism produces the light that we catch with our high-energy satellites, and most of all, what, if anything, is the role and origin of magnetic fields. Our results are important because they show that magnetic fields are present close to the central black hole, are threaded through the material that is ejected at ultra-high speeds in the explosion and ultimately focus and accelerate the material to large distances from the black hole.”

The data also suggest that synchrotron radiation—which results when electrons are accelerated in a curved or spiral pathway—powers the initial, extremely bright phase of the burst, known as the “prompt” phase. Until now there had been several other possible explanations but these results give a clearer picture of what physical conditions create GRBs.

Dr Eleonora Troja, an assistant research scientist in the University of Maryland and lead author of the research paper, said: “Gamma-ray bursts are catastrophic events, related to the explosion of massive stars 50 times the size of our sun. If you ranked all the explosions in the universe based on their power, GRBs would be right behind the Big Bang. In a matter of seconds, the process can emit as much energy as a star the size of our Sun would in its entire lifetime. We are very interested to learn how this is possible.

“Our study provides convincing evidence that the prompt GRB emission is driven by synchrotron radiation. This is an important achievement because, despite decades of investigation, the physical mechanism that drives GRBs had not yet been unambiguously identified.”

This research was supported by the UK Space Agency, NASA, El Consejo Nacional de Ciencia y Tecnología, Mexico, the National Autonomous University of Mexico, the University of California Institute for Mexico and the United States-El Consejo Nacional de Ciencia y Tecnología Collaborative Grants Program, and the Russian Science Foundation.

Significant and variable linear polarization during the prompt optical flash of GRB 160625B” by Eleonora Troja et al., is published in the journal Nature, doi:10.1038/nature23289

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