Using NASA’s X-ray telescope mounted on the International Space Station (ISS), astronomers have considered the rapidly rotating dead star that represents the heart of the closest millisecond pulsar to Earth.
Like all neutron stars, pulsars are born when massive stars die, but what really sets millisecond pulsars apart is the fact that they rotate hundreds of times per second. As they do, beams of radiation and matter shoot out from the poles of these dead stars and spread through space, making pulsars resemble powerful “cosmic beacons.”
Located about 510 light-years from Earth in the constellation Pictor, 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 rotates 174 times per second, meaning that every 5.75 milliseconds it pummels Earth with X-ray and radio waves. These pulses are so regular that, like other pulsars, this fast cosmic beacon may actually be used to keep time.
Scientists now know that the neutron star that makes up PSR J0437 is 14 miles (22.5 kilometers) across and has a mass equivalent to 1.4 times the mass of the Sun. The team also found that the neutron star’s hot magnetic poles are misaligned and not directly opposite each other.
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To collect new measurements of PSR J0437, the team turned to NASA’s Neutron Star Interior Composition Explorer (NICER), which is attached to the ISS. They processed this X-ray data using a modeling method called “pulse profile modeling” and then created simulations of PSR J0437 using the Dutch National Supercomputer Snellius.
“Before, we hoped to be able to calculate the radius precisely. And it would be great if we could prove that the hot magnetic poles are not directly opposite each other on the stellar surface,” team leader Devarshi Choudhury of the University of Amsterdam. he said in a statement. “And we just did both!”
The most extreme stars
When stars with masses between eight and 25 times the mass of the Sun run out of fuel, they can no longer conduct nuclear fusion in their cores after billions of years of existence. This not only cuts off most of the energy the star emits, but also stops the outward flow of radiation pressure.
During the life of a star, this radiation pressure supports it against the internal pressure of its own gravity. This means that when its fuel is exhausted, the star can no longer resist gravitational collapse. As the core collapses, this process sends shock waves through the star’s outer layers, triggering a supernova explosion that rips off most of the star’s mass. Regardless of the star’s initial mass, the resulting neutron star would be born with a much narrower range of masses, from one to twice the mass of the Sun.
However, the collapse of this dying stellar core reduces the width of the progenitor neutron star to about 20 kilometers. As a result, the matter that contains the neutron star is so dense that a sugar cube from it brought to Earth would weigh 1 billion tons—about 2,500 times the weight of the Empire State Building.
There is another consequence of the rapid shrinking of the star’s core to form a neutron star. Due to the conservation of angular momentum, the radical reduction of the radius causes an increase in the rotation rate of the stellar remnant. It’s similar to how skaters on Earth draw in their arms to increase the speed of a pirouette.
Neutron stars that form pulsars can also get an extra speed boost from a companion star. When a neutron star and a companion star are close enough, the former can separate material from the latter. This stellar mass carries with it an angular momentum that further increases the rate of rotation of the neutron star.
PSR J0437 may have engaged in this stellar cannibalism in the past to achieve a rotation rate of 174 revolutions per second. This is evidenced by the fact that it has a helium-rich white dwarf companion with a mass of only a quarter of the Sun’s, which appears to have had its outer layers stripped away.
While many measurements of PSR J0437 have confirmed that scientists understand how these objects form, this millisecond pulsar has yielded one surprise. The team’s mass of PSR J0437 suggests that the maximum mass of neutron stars could be lower than some current theories predict.
“This again fits well with what the gravitational wave observations suggest,” said team member and neutron star expert at the University of Amsterdam, Anna Watts.
The team’s research was published in a series of peer-reviewed articles on the arXiv website and in the Astrophysical Journal.