A dead star 27,400 light-years away appears to be having one of the most epic stellar discos we've ever seen.
In the binary system 4U 1820-30, a neutron star is spinning
so fast around its center axis that it completes a breathtaking 716 rotations
per second. No stars have been discovered spinning faster, and only the famous
pulsar PSR J1748-2446ad has been found spinning at that speed.
This discovery, says a team led by astrophysicist Gaurava
Jaisawal of the Technical University of Denmark, affirms the current
theoretical upper limit for neutron star spin speeds, thought to be around 730
rotations per second.
"We were studying thermonuclear explosions from this
system and then found remarkable oscillations," Jaisawal says.
"If future observations confirm this, the 4U 1820-30
neutron star would be one of the fastest-spinning objects ever observed in the
Universe."
Neutron stars are one of the evolutionary final stages in
the lifespan of a massive star.
Once a star between around 8 and 30 times the mass of the
Sun has run out of atoms it can fuse in its core; it goes kaboom (or maybe it
doesn't), ejecting its outer material in a supernova explosion, while the core,
no longer supported by the outward pressure supplied by fusion, collapses under
gravity.
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An artist's impression of a bursting X-ray pulsar. (Chris
Smith/Walt Feimer/NASA's Goddard Space Flight Center) |
That collapsed core is the neutron star, an object between
around 1.1 and 2.3 times the mass of the Sun, packed into a tiny sphere just 20
kilometers (12 miles) across. 'Dense' doesn't begin to cover it. Matter inside
these things can get weird – and so can their behavior.
We have different names to classify this behavior. A
magnetar is a neutron star with an insanely powerful magnetic sphere. And a
pulsar is a neutron star spinning incredibly fast, blasting out beams of radio
waves from their poles so they appear to 'pulse' in space like a cosmic
lighthouse.
We've known 4U 1820-30 exists since at least the 1980s. It's
a binary star system in the constellation of Sagittarius that includes a
neutron star and a white dwarf star, on a really tight orbit with a period of
just 11.4 minutes.
The proximity of the two stars means the neutron star is
close enough to cannibalize its companion, stripping it of material that then
accumulates on the neutron star's surface.
This accumulation of mass becomes denser and hotter the more
it accrues, until eventually the star sneezes it off in a thermonuclear
explosion.
It was these explosions that Jaisawal and his colleagues
were trying to study, using NASA's Neutron Star Interior Composition Explorer
(NICER), an X-ray telescope catching a ride with the International Space
Station.
"During these bursts, the neutron star becomes up to
100,000 times brighter than the sun, releasing an immense amount of
energy," says astrophysicist Jerome Chenevez of the Technical University
of Denmark.
"So we are dealing with very extreme events, and by
studying them, we get new insights into the existing life cycles of binary star
systems and the formation of elements in the Universe."
The team recorded 15 of these thermonuclear blasts between
2017 and 2022. But when they were analyzing the data, they found something
strange. One of the blasts had an odd signature, an oscillation with a
frequency of 716 Hertz. It was as though the star was rotating as it erupted –
which, the researchers determined, was likely the case.
This could mean that the 4U 1820-30 neutron star is an X-ray
pulsar, with a period of 716 rotations per minute, powered by thermonuclear
explosions. Since PSR J1748-2446ad is a radio pulsar, this would make 4U
1820-30 the fastest known nuclear-powered pulsar.
More observations will be required to verify this finding.
If confirmed, however, the results will give us a new tool for studying neutron
stars, and the extremities they can reach before they self-destruct.
The team's research has been published in The Astrophysical Journal.