Astronomers Glimpse Infrastructure of Energetic Gamma-Ray Burst's High-Speed Jet

Gamma-ray bursts are some of the most energetic explosions in the universe. Now, astronomers are getting a closer look at the magnetic fields at the heart of these bursts. They've glimpsed the infrastructure of a burst's high-speed jet, revealing a bit more about this space phenomenon.

Gamma-ray bursts are the most luminous explosions in the cosmos. Most of these bursts are thought to be triggered when the core of a massive star runs out of nuclear fuel, collapses under its own weight and forms a black hole. This black hole then drives jets of particles that drill all the way through the collapsing star and erupt into space at nearly the speed of light.

On March 8, 2012, the scientists spied such a gamma-ray burst, dubbed GRB 120308A, in the constellation Ursa Minor. This jet created a bright afterglow, which the researchers measured and studied.

Energy across the spectrum, from radio ways to gamma rays, is emitted when a jet slams into its surroundings and begins to decelerate. This results in the formation of an outward-moving shockwave. At the same time, a reverse shockwave drives back into the jet debris and also produces a bright emission.

"One way to picture these different shocks is by imagining a traffic jam," said Carole Mundell, one of the researchers, in a news release. "Cars approaching the jam abruptly slow down, which is similar to what happens in the forward shock. Cars behind them slow in turn, resulting in a wave of brake lights that moves backward along the highway, much like the reserves shock."

Models of gamma-ray bursts predict that light from this reverse shock should show strong and stable polarized emissions if the jet possesses a structured magnetic field originating from the environment around the newly-formed black hole. Previous observations of optical afterglows have also shown that there are polarizations about 10 percent of the time; yet these observations provided no information about how the value changed with time, which means they could not be used to test competing jet models.

In this case, though, the new findings have enabled researchers to catch the explosion just four minutes after the initial outburst. The new data supports the presence of a large-scale organized magnetic field linked to the black hole, rather than a tangled magnetic field produced by instabilities within the jet itself.

The findings reveal a little bit more about gamma-ray bursts. This, in turn, will help inform future studies and confirm theoretical models.

The findings are published in the journal Nature.