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Polarized light in a gamma-ray burst’s jet linked to a magnetic field (a blue spiral, in this illustration) originating from the newly-formed black hole. Image credit NASA s Goddard Space Flight Center S.Wissinger Polarized light in a gamma-ray burst’s jet linked to a magnetic field (a blue spiral, in this illustration) originating from the newly-formed black hole. Image credit NASA's Goddard Space Flight Center S.Wissinger

Measuring the infrastructure of a gamma-ray burst's high-speed jet

Sen— The world's largest fully autonomous robotic optical telescope has provided the best look to date at magnetic fields at the heart of gamma-ray bursts.

Gamma-ray bursts are the most energetic explosions in the Universe. Most 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. The black hole drives jets of particles that drill through the collapsing star and erupt into space at nearly the speed of light.

On March 8, 2012, NASA’s Swift satellite detected a 100-second pulse of gamma rays from a source in the constellation Ursa Minor. The spacecraft immediately forwarded the location of the gamma-ray burst, dubbed GRB 120308A, to observatories around the globe.

The 2-metre Liverpool Telescope at Roque de los Muchachos Observatory on La Palma in the Canary Islands, automatically responded to Swift's notification.

"Just four minutes after it received Swift's trigger, the telescope found the burst's visible afterglow and began making thousands of measurements," said lead researcher Carole Mundell, who heads the gamma-ray burst team at the Astrophysics Research Institute at Liverpool John Moores University in the U.K.

An international team of astronomers from Britain, Slovenia and Italy used an instrument named RINGO2, fitted to the telescope, to probe the magnetic fields long postulated to drive and focus the jets of gamma-ray bursts. The shoe-box-sized instrument pairs a spinning polarizing filter with a super-fast camera. It was designed by Mundell's team to detect any polarization, in the vibration of light waves from burst afterglows.

Energy, from radio waves 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 shock wave. At the same time, a reverse shock wave drives back into the jet debris, also producing bright emission.

Theoretical models predict that light from the 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, thought to be the "central engine" driving the burst.

Previous observations of optical afterglows detected polarizations of about 10 percent, but they provided no information about how this value changed with time.

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. Image Credit: NASA's Goddard Space Flight Center/S. Wiessinger.

The Liverpool Telescope caught the explosion just four minutes after the initial outburst. Over the following 10 minutes, RINGO2 collected 5,600 photographs of the burst afterglow while the properties of the magnetic field were still encoded in its captured light.

The observations show the initial afterglow light was polarized by 28 percent, the highest value ever recorded for a burst, and slowly declined to 16 percent, while the angle of the polarized light remained the same. This 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.

"This is a remarkable discovery that could not have occurred without the lickety-split response times of the Swift satellite and the Liverpool Telescope," said Neil Gehrels, the Swift principal investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

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