One of the weirdest stars in the Milky Way has just gotten even weirder.
It's a magnetar called XTE J1810-197, and it was discovered
in 2003 just riotously spitting out radio waves. In 2008, it just… stopped, and
pretty much vanished from our view.
Then, in 2018, it roused again. Only this time, there was
something odd about the radiation it was emitting.
Now, scientists have analyzed those radio waves in two
separate papers, and the results are wack. Those low frequency electromagnetic
emissions are twisted in a way we haven't seen before, and the star is
appearing to wobble.
"Unlike
the radio signals we've seen from other magnetars, this one is emitting
enormous amounts of rapidly changing circular polarization," says astrophysicist Marcus Lower of the CSIRO in Australia, who led the first paper.
"We had never seen anything like this before."
"We expected to see some variations in the polarization
of this magnetar's emission, as we knew this from other magnetars,"
explains Gregory Desvignes from the Max Planck Institute for Radio Astronomy
(MPIfR) in Germany, who led the second paper.
"But we did not expect that these variations are so
systematic, following exactly the behavior that would be caused by the wobbling
of the star."
All stars are special in their own way, but magnetars are
possibly the weirdest of the weird. They're very young neutron stars, which
themselves are the collapsed cores of dead massive stars that have gone
supernova and ejected most of their material in a colossal explosion. The core
that remains collapses under gravity, and it is dense – up to 2.3 times the
mass of the Sun, squished into a ball just 20 kilometers (12 miles) across.
Following this collapse process, neutron stars briefly
possess an insanely powerful magnetic field. They're basically the most
magnetic things in the Universe, with magnetic fields 1,000 times more powerful
than a normal neutron star's, and a quadrillion times more powerful than
Earth's.
That makes them behave a bit strangely. For example,
scientists think that the constant tug-of-war waged between the magnetic field
and the gravity of a magnetar causes it to erupt in giant quakes from time to
time, sending out blasts of radio waves we call fast radio bursts.
So, just being a magnetar makes XTE J1810-197 an oddball,
even without the on-and-off-and-on-again monkeyshines. The recent activity,
however, is unprecedented – and could provide new insight into these mysterious
stars.
Lower, Desvignes, and their colleagues measured a property
of the light emitted by the star known as polarization. That's when the
oscillation of the light that reaches us is oriented in a preferred direction.
It's normal for magnetar light to be polarized, rotated by the powerful
magnetic field it has to pass through to reach us.
Normally, magnetars emit mostly linear polarized light, with
a small amount of circularly polarized light traveling in a spiral pattern. XTE
J1810-197, Lower and his team found, was emitting huge amounts of circularly
polarized light.
Theory suggests that this can happen when the light has to
travel through a thick, super-heated 'soup' of particles that can be found in
the magnetic field of a neutron star. The behavior of XTE J1810-197 doesn't
quite match this prediction, but the researchers have some ideas.
"Our results suggest there is a superheated plasma
above the magnetar's magnetic pole, which is acting like a polarizing
filter," Lower says. "How
exactly the plasma is doing this is still to be determined."
Meanwhile, Desvignes and his team found that the
polarization revealed a shift in the magnetar's orientation with respect to
Earth. In other words, it seemed to be wobbling, or precessing, like a spinning
top. But this got surprising, too: over the next few months, the precession
damped significantly, and eventually stopped entirely.
The researchers believe this might be because of a rupture
in the surface of the star. This could cause it to temporarily wobble, and also
produce super-heated particles in the magnetic field. But if this is the case,
and is also normal for magnetars, then it may put paid to the theory that
precessing magnetars spew out the rare, repeated fast radio bursts we
occasionally detect.
On the other hand, the behavior might reveal something new
about some of the most extreme objects in the Universe.
"Damped precession of magnetars might shed light on the
inner structure of neutron stars, which is ultimately related to our
fundamental understanding of matters," says astrophysicist Lijing Shao of
Peking University.
We'll just have to keep watching to see what these wacky
dead stars come up with next.
Reference(s): Research Paper 1, Research paper 2
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