3/7/2025 Bill Bell for Illinois Physics
Research by Frederick Lamb and colleagues gives a precise measurements of an important neutron star and provides a better understanding of the most dense matter known in the universe.
Written by Bill Bell for Illinois Physics
Far from the center of the Milky Way, roughly 4,000 light years from Earth, sits PSR J0740+6620. It’s a distinctly interesting neutron star, the collapsed core left behind when a supergiant exploded, creating a supernova. It completes one rotation every 2.9 milliseconds, throwing off X-rays and other electromagnetic radiation with astonishing consistency, making it one of only about 3,000 pulsars identified to date.
But there’s another thing that makes PSR J0740+6620 fascinating. It’s the neutron star with the highest precisely determined mass known by astronomers. Its mass is thought to be about 2.1 times the mass of our Sun, yet its radius is less than that of a major city. (For comparison, the radius of the Sun is about ten times that of Jupiter, and the radius of Jupiter is about 11 times that of Earth.)
So much mass in an object so small makes the matter in PSR J0740+6620’s core the most dense matter known in the universe.
This type of matter “certainly can’t be produced in a laboratory,” according to Illinois Physics Professor Frederick Lamb, who began his career at Illinois in 1970. “Some aspects are seen in particle colliders at CERN or Fermilab, but that matter has a much higher temperature and more protons and other particles. The matter in the core of a neutron star has some similar properties, but it’s not the same stuff.”
The matter in a neutron star core is many thousands of times cooler than the matter created in a heavy particle collider.
“There’s nothing else like it in the universe,” says Lamb.
Core issues
To better understand a star, you’d better understand the matter at its core. An October 2024 paper in The Astrophysical Journal, authored by Lamb and longtime collaborators from around the world, significantly refined our perspective on PSR J0740+6620. It gave a more precise measurement of the star’s radius and showed that the matter in its core is softer than previously thought.
The new findings are based on a method first proposed and developed in 2013 by Lamb, then-Illinois graduate student Ka Ho Lo, and Coleman Miller, a former postdoctoral student of Lamb’s who is a professor of astronomy at the University of Maryland. Miller was a co-author of the 2024 journal article, and its first author was Alex Dittmann, an Illinois alumnus in undergraduate physics and astronomy. Another group also used this method to provide an independent and consistent estimate of the star’s radius.
The method uses measurements of the mass and radius of a neutron star to determine the equation of state of the matter in its core—how soft or stiff the matter is. These measurements rely on two key effects. The first is the Doppler shift of the X-rays emitted from the star, caused by the rapid rotation of its surface. The second is the bending of the paths the X-rays follow when leaving the star, caused by its gravitational mass. Once the star’s mass and radius are known, the stiffness of the matter in its core can be inferred.
“A variety of other methods to determine the equation of state have been tried, but they are challenged by systematic errors. If there’s a systematic error, there’s something wrong with your model that you do not understand,” Lamb explains.
“Unlike those methods, it appears that using this method, if you are able to fit the data well, then the properties of the star you infer are not systematically wrong. Consequently, the uncertainty about these properties can be reduced by using more data.”
The Astrophysical Journal article showed that the team’s understanding of dense matter has improved using the additional data that has been accumulated during the last several years.
“I was skeptical, and many other people were too. And there are still many challenges,” Lamb notes. “But it does appear that additional measurements will further constrain the equation of state of the ultradense matter in the cores of neutron stars.”
None NICER
The measurements were made using the Neutron Star Interior Composition Explorer (NICER), which was built by NASA and launched to the International Space Station in 2017.
NICER is being used to study neutron stars, black holes, and other phenomena. An array of 56 X-ray detectors records each photon’s energy and time of arrival to an accuracy of 300 nanoseconds, accumulating a million or more X-ray photons from each star.
NICER data were used to determine the radius of J0740+6620 and thus the equation of state of the matter in its core.
Lamb and the rest of the 80-member NICER team were awarded the 2022 Rossi Prize of the American Astronomical Society, considered the top award in high-energy astrophysics. The Rossi Prize citation specifically mentioned “the revolutionary insights NICER is providing about the extreme environments of neutron stars and black holes, including the first precise and reliable measurement of a pulsar’s mass and radius from detailed modeling.”
That first measurement followed from Lamb and Miller’s work a decade ago, as have the other measurements that have been made since.
Lamb credits the instrumentalists who built NICER and made it work on a relatively small budget. For the first time, the team used solid-state photon detectors, rather than traditional gas detectors, to measure cosmic X-rays.
“It was a pioneering effort to make this kind of detector, and it worked fabulously well,” Lamb says. “This is the wave of the future.” Using this method to determine the equation of state of the matter in the cores of neutron stars and these very reliable, high-speed detectors in years to come, astrophysicists will be able to further refine what we know about this ultradense matter.
Already, the team’s research on PSR J0740+6620 has revealed important information about intriguing properties of dense matter, such as its composition, at what density quarks begin to appear, and the properties of quark matter.
“We still don’t fully understand how ordinary nuclei become quark matter as the density increases. Determining the properties of the matter in the cores of these stars is improving our understanding,” Lamb concludes.
Support for this research was provided by NASA and the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the scientists and do not necessarily reflect the views of the funding agencies.