Gamma-ray bursts scramble their magnetic fields
The find could help astronomers understand what makes these events so energetic
Gamma-ray bursts (GRBs) scramble their magnetic fields within minutes, a new study has found, confirming a decade-old theory about how these explosions are created.
GRBs are rapid eruptions of energy created when a massive star – at least 40 times larger than the Sun – reaches the end of its life and collapses to form a black hole. These extreme events throw out stellar material at velocities approaching the speed of light, which creates a short-lived flash of gamma radiation we then detect here on Earth.
It’s thought that magnetic fields play a critical role in making GRBs so extreme, but exactly what role is unclear. The theory is that these fields thread themselves through the material thrown out by the collapsing star, but these soon become twisted by the spinning of the newly created black hole. Then, as the advancing debris crashes into the surrounding interstellar material, the magnetic fields get disrupted and eventually destroyed.
Testing this theory, however, has proven difficult. It is impossible to see these magnetic fields directly, meaning that astronomers have to look for the hallmarks they leave behind in the light given off by the gas they run through.
“We measured a special property of the light – polarisation – to directly probe the physical properties of the magnetic field powering the explosion,” says Carole Mundell from the University of Bath, who led the study.
However, these observations require a different type of telescope to the kind that can spot GRBs in the first place. There is only a short time frame to turn a follow-up telescope onto the position of a burst before the magnetic fields are destroyed and there is no more polarised light to observe.
But Mundell’s team made arrangements for the robotic Liverpool Telescope in La Palma to automatically slew to the location of any new GRB as soon as it was spotted, and managed to get follow-up data just 90 seconds after the gamma radiation reached Earth. Using this method, they could track how the magnetic fields moved and decayed after the explosion, finding they were rapidly obliterated in the minutes following the initial blast.
“This is a great result and solves a long-standing puzzle of these extreme cosmic blasts – a puzzle I’ve been studying for a long time,” says Mundell. www.bath.ac.uk