Two neutron stars. One is 50% more massive than the other, but they are almost exactly the same size. The results have a major impact on understanding what neutron stars are really made of.
In 2019, a team of astronomers led by postdoc Thomas Riley and professor of astrophysics Anna Watts at the University of Amsterdam measured the mass and radius of a particular neutron star. The neutron star in question was J0030 + 0451 (or J0030 for short). They found it to be about 1.4 times the mass of the Sun and about 16 miles in diameter – pretty typical of neutron stars.
For the measurement, the team used the neutron star Interior Composition Explorer (NICER) from NASA, an X-ray telescope on the International Space Station. J0030 is a special type of neutron star called a pulsar that emits powerful rays of radiation into space. By carefully studying the frequency of the jets, the team was able to estimate its size.
More recently, they applied the same technique to the neutron star PSR J0740 + 6620 (J0740 for short), the most massive neutron star known.
Although J0740 weighed 50% more than J0030, it was almost exactly the same size. The new results suggest that neutron stars aren’t very mushy: when they add mass, they don’t tend to squeeze onto smaller volumes.
“Our new measurements on J0740 show that while it is nearly 50% more massive than J0030, it is essentially the same size,” said Watts. “This challenges some of the more squeezable models of neutron star nuclei, including versions where the inside is just a sea of quarks. The size and mass of the J0740 also pose problems for some less squeezable models that only contain neutrons and protons. “
Physicists have long struggled to understand the interior of neutron stars.
“We are surrounded by normal matter, the stuff of our daily experience, but we don’t know much about how matter behaves and how it changes under extreme conditions,” said Zaven Arzoumanian, the scientific director of NICER at NASA’s Goddard Space Center in Greenbelt, Maryland. “By measuring the sizes and masses of neutron stars with NICER, we are studying matter that is about to implode into a black hole. Once this happens, we can no longer study the matter because it is obscured by the black hole’s event horizon. “
While the new observations rule out some models of neutron star interiors, there is still much to be done.
“The size of J0740 has amazed and excited us theorists,” said Sanjay Reddy, a professor of physics at the University of Washington, who studied matter under extreme conditions but was not involved in the discovery. “NICER’s measurements in combination with other multimessenger observations seem to support the idea that the pressure in massive neutron star nuclei increases rapidly. While this favors transitions to more squeezable forms of matter in the core, its implications have yet to be fully understood. “
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