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Using NASA’s X-ray telescope aboard the International Space Station (ISS), astronomers have measured a rapidly spinning dead star that represents the heart of the millisecond pulsar closest to Earth.
Like all neutron stars, pulsars are born when the massive stars die, but what really sets millisecond pulsars apart is that they spin hundreds of times per second. As they do this, beams of radiation and matter burst out of the poles of these dead stars and sweep across the universe, making pulsars like powerful “cosmic lighthouses”.
Located about 510 light-years from Earth in the constellation Picturi, PSR J0437-4715 (PSR J0437) is the closest example of a millisecond pulsar to our solar system and the brightest example of such an object in the night sky. PSR J0437 spins 174 times per second, meaning it blasts Earth with X-rays and radio waves every 5.75 milliseconds. These pulses are so regular that this fast cosmic beacon can be used to keep time, like other pulses.
Now, scientists know that the neutron star that forms PSR J0437 is 14 miles (22.5 kilometers) across and has a mass equal to 1.4 times that of the Sun. The team also discovered that the neutron star’s hot magnetic poles are misaligned and not directly opposite each other.
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To collect the new PSR J0437 measurements, the team turned to the Neutron Star Interior Composition Explorer (NICER) attached to the ISS. They treated this X-ray data with a modeling method called “pulse profile modeling” and then created simulations of PSR J0437 using the Dutch Snellius supercomputer.
“Before, we expected to be able to calculate the radius accurately. And it would be great if we could show that the hot magnetic poles are not directly opposite each other on the stellar surface,” the team leader Devarshi Choudhury of the University of Amsterdam. said in a statement. “And we managed to do both!”
The most intense stars
When stars with eight to 25 times the mass of the sun run out of fuel, after billions of years of life, they can no longer do nuclear fusion at their cores. This not only cuts off most of the energy radiated by a star, but stops the outflow of radiation pressure.
Over a star’s lifetime, this radiation pressure supports it against the inward pressure of its own gravity. That means that when its fuel is exhausted, the star can no longer prevent itself from gravitational collapse. As the core collapses, this process sends shock waves through the star’s outer layers, triggering a supernova explosion that destroys most of the star’s mass. Regardless of the star’s initial mass, a resulting neutron star would be produced with a much tighter range of masses, from one to twice the mass of the Sun.
However, the collapse of this dying star’s core reduces the width of the progenitor neutron star to about 12 miles (20 kilometers). Therefore, the material that comprises a neutron star is so dense that a sugar cube brought to Earth would weigh 1 billion tons — that’s about 2,500 times the weight of the Empire State Building.
Another consequence is the rapid contraction of a stellar core to give birth to a neutron star. Due to the conservation of angular momentum, the decreasing radial radius increases the spin speed of the stellar remnant. This is like an ice skater on Earth pulling in their arms to increase the speed of a pirouette.
Neutron stars that form pulses can get an extra speed boost from a companion star. When the neutron star and its stellar companion are close enough, the former can remove material from the latter. This stellar material carries with it angular momentum, further increasing the neutron star’s rotational speed.
PSR J0437 may have been involved in this stellar cannibalism in the past to achieve its spin speeds of 174 rotations per second. The evidence for this is that it has a helium-rich white companion star that is only a quarter of the size of the sun, and its outer layers appear to have been stripped away.
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While many of the measurements in PSR J0437 have confirmed scientists’ understanding of how these objects form, this millisecond pulsar provided one surprise. The mass of PSR J0437 suggests to the team that the maximum mass of neutron stars may be lower than some theories currently predict.
“That, in turn, fits perfectly with the observations that gravitational waves imply,” said Anna Watts, a team member and neutron star expert at the University of Amsterdam.
The team’s research has been published in a series of peer-reviewed papers on the task site arXiv and in the Astrophysical Journal.
