Every day or two, on average, satellites detect
a massive explosion somewhere in the sky.
These are gamma-ray bursts, the brightest
blasts in the universe. They're thought to
be caused by jets of matter moving near the
speed of light associated with the births
of black holes. Gamma-ray bursts that last
longer than two seconds are the most common
and are thought to result from the death of
a massive star. Shorter bursts proved much
more elusive.
In fact, even some of their basic properties
were unknown until NASA's Swift satellite
began work in 2004. A neutron star is what
remains when a star several times the mass
of the sun collapses and explodes. With more
than the sun's mass packed in a sphere less
than 18 miles across, these objects are incredibly
dense. Just a sugar-cube-size piece of neutron
star can weigh as much as all the water in
the Great Lakes.
When two orbiting neutron stars collide, they
merge and form a black hole, releasing enormous
amounts of energy in the process. Armed with
state-of-the-art supercomputer models, scientists
have shown that colliding neutron stars can
produce the energetic jet required for a gamma-ray
burst. Earlier simulations demonstrated that
mergers could make black holes. Others had
shown that the high-speed particle jets needed
to make a gamma-ray burst would continue if
placed in the swirling wreckage of a recent
merger.
Now, the simulations reveal the middle step
of the process --how the merging stars' magnetic
field organizes itself into outwardly directed
components capable of forming a jet. The Damiana
supercomputer at Germany's Max Planck Institute
for Gravitational Physics needed six weeks
to reveal the details of a process that unfolds
in just 35 thousandths of a second. The new
simulation shows two neutron stars merging
to form a black hole surrounded by super-hot
plasma.
On the left is a map of the density of the
stars as they scramble their matter into a
dense, hot cloud of swirling debris. On the
right is a map of the magnetic fields, with
blue representing magnetic strength a billion
times greater than the sun's. The simulation
shows the same disorderly behavior of the
matter and magnetic fields. Both structures
gradually become more organized, but what's
important here is the white magnetic field.
Amidst this incredible turmoil, the white
field has taken on the character of a jet,
although no matter is flowing through it when
the simulation ends.
Showing that magnetic fields suddenly become
organized as jets provides scientists with
the missing link. It confirms that merging
neutron stars can indeed produce short gamma-ray
bursts. At this moment, somewhere across the
cosmos, it's about to happen again.
