Astronomers have found the first evidence of cracks in a
neutron star's crust. The star cracked when it was rocked by the strongest
"starquake" ever recorded, researchers said
last week.
Last December, astronomers worldwide monitored the explosion
that caused this starquake. The eruption was huge – in the first
200 milliseconds of the event the star released energy equivalent to what our
Sun produces in 250,000 years. The explosion was the brightest event ever
detected outside of our Solar System.
Now scientists have used a collection of data from various
satellites to provide the first observational evidence that the blast caused
the star to form cracks several miles long. Scientists hope these cracks will
provide a window into the mysterious interiors of neutron stars.
There are millions of neutron stars in the Milky Way galaxy
alone, and some of these have magnetic fields trillions of times stronger than
Earth's, the strongest of which are called magnetars.
This particular magnetar – SGR
1806-20 – is surrounded by the strongest magnetic field known in the
universe. This could explain why the starquake
– caused when the magnetar's crust could no
longer contain the magnetic stress building in the star's interior – was
so intense.
A magnetar's interior is a dense,
liquid-like mix of neutrons, protons, and electrons – making it a
terrific conductor of electricity. Because it has the characteristics of a
fluid, it moves around a lot. The magnetar's magnetic
field loops around the star, and all this movement in the interior messes with
the field's shape, winding it up like you might do with a rubber band.
But the magnetar's exterior crust is
not so pliable. The crust is made mostly of iron. The magnetic field passes
through it in places, which isn't a problem for normal neutron stars. But in magnetars, the field interacts with the core and shifts
around erratically, causing crustal stress.
Eventually, the stress reaches the point where the crust cracks.
"Imagine threading a rubber band between two cards, and
then twisting the middle," study leader Steve Schwartz of the Imperial
College of London told SPACE.com.
"All those twisting stresses accumulate at the points where the rubber
band threads through the card to the outside. Keep twisting long enough and you
will rip the card."
The first crack to form was three miles (five kilometers long)
– significant since this magnetar is only six
miles (10 kilometers) in diameter. Radiation spewed from this crack, causing a
steep initial increase in detectable radiation.
But that was just the beginning. Radiation continued to spill
out of the star, but at a much slower rate than the initial burst. This
suggests that cracks continued to form.
"Whether this is a set of long, [three mile] cracks, or a
multitude of much smaller ones isn't obvious to me," Schwartz said.
"My hunch is therefore: one big one, followed by lots and lots of ongoing
smaller ones."
What this means for SGR 1806-20 isn't clear, but it seems that
cracks form more to relieve pressure than as a sign that the star is blowing
apart.
"The result of the cracking is to relax the interior and
exterior field to a less twisted state," Schwartz said. "This has
very little impact on the star itself, other than the fact that it will take
time to twist up the field again."
SGR 1806-20 is 50,000 light-years away, but the blast was so huge
it temporarily blinded some satellites and briefly
altered Earth's upper atmosphere. A similar blast occurring within 10
light-years of our planet would fry Earth's ozone layer. But don't worry
– the closest magnetar is 13,000 light-years
away.
Two satellites designed to study the Earth's magnetosphere
– the European Space Agency's Cluster and Double Star satellites –
didn't go offline and recorded the entire event. Data from these two satellites
was combined with observations from around the world to uncover the cracks.
So far, nine magnetars have been
firmly identified, and four of these repeatedly emit bursts of X-rays and gamma
rays. SGR 1806-20, which has a magnetic field more powerful than any other
object in the universe, is one of these so-called soft
gamma repeaters.
Researchers still don't know why SGR 1806-20's burst was so
incredible, but they hope that a look into its cracks will help solve the
mystery.
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