A new study using observations from a novel
instrument provides the best look to date at magnetic fields
at the heart of gamma-ray bursts, the most energetic
explosions in the universe. An international team of
astronomers from Britain, Slovenia and Italy has glimpsed
the infrastructure of a burst's high-speed jet.
Gamma-ray bursts are the most
luminous explosions in the cosmos.
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 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, 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 world's largest fully autonomous robotic optical
telescope, the 2-meter Liverpool Telescope located 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.
The telescope was fitted with an instrument named RINGO2,
which Mundell's team designed to detect any preferred
direction, called polarization, in the vibration of light waves
from burst afterglows.
Mundell's team built RINGO2 in order 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.
Energy across the spectrum, 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.
"One way to picture these different shocks is by imagining a
traffic jam," Mundell said. "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 reverse shock."
Theoretical models of gamma-ray bursts 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. As a
result, they could not be used to test competing jet models.
The Liverpool Telescope's rapid targeting enabled the team
to catch 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 that 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.
A paper describing the team's findings will appear in the
Dec. 5 issue of the journal Nature.
"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, Md.
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