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More Insight into Neutron Star Interiors

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Illustration of a compact star with beams of light emitting from its poles.
What does the inside of a neutron star look like? [NASA’s Goddard Space Flight Center Conceptual Image Lab]

Editor’s note: AAS Nova is on vacation until 22 September. Normal posting will resume at that time; in the meantime, we’ll be taking this opportunity to look at a few interesting AAS journal articles that have recently been in the news or drawn attention.

What’s inside the dense interior of a neutron star, the remnant left behind at the end of a massive star’s evolution? Scientists have now searched for the answer to this question using new observations of an extreme neutron star from NASA’s Neutron star Interior Composition Explorer (NICER).

schematic illustrating the different layers of a neutron star, including an unknown inner core

Scientists think neutron stars are layered. As shown in this illustration, the state of matter in their inner cores remains mysterious. [NASA’s Goddard Space Flight Center/Conceptual Image Lab]

Neutron star J0740+6620 is the heaviest neutron star that’s been precisely measured — and correspondingly precise measurements of its radius could provide the key to finally figuring out what its interior structure looks like. In a set of recent studies, one led by Cole Miller (University of Maryland) and the other by Thomas Riley (University of Amsterdam), two teams of scientists used NICER’s X-ray observations of J0740 to obtain accurate measures of the star’s radius using two different approaches. They found that J0740’s 2.1 solar masses are packed into a sphere just 25–27 km across.

These new results, combined with previous measurements of other neutron stars, are helping us to understand whether neutron stars are made up primarily of neutrons in their interior, or whether the pressure is so great that those neutrons have disintegrated into a soup of particles called quarks. A study led by Geert Raaijmakers (University of Amsterdam) uses these observations to place significant constraints on the so-called neutron star equation of state, which describes neutron star interiors.

To learn even more about this work, be sure to check out the summary video from NASA’s Goddard Space Flight Center below.

Original articles:
“The Radius of PSR J0740+6620 from NICER and XMM-Newton Data,” M. C. Miller et al 2021 ApJL 918 L28. doi:10.3847/2041-8213/ac089b
“A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-Newton Spectroscopy,” Thomas E. Riley et al 2021 ApJL 918 L27. doi:10.3847/2041-8213/ac0a81
“Constraints on the Dense Matter Equation of State and Neutron Star Properties from NICER’s Mass–Radius Estimate of PSR J0740+6620 and Multimessenger Observations,” G. Raaijmakers et al 2021 ApJL 918 L29. doi:10.3847/2041-8213/ac089a

Press releases:
University of Maryland: NASA’s NICER Probes the ‘Squeezability’ of Neutron Stars
University of Amsterdam: Astronomers Measure Heaviest Known Neutron Star With Telescope on ISS