NICER / ISS Science Nugget
for June 5, 2025

NICER and fundamental nuclear physics

The original "key science" objective of the NICER mission has been a deeper understanding of the nature of matter in the cores of neutron stars. At the threshold of collapse into a black hole event horizon, this ultra-dense stuff represents the final stable endpoint of matter allowed by nature, beyond which a collection of subatomic particles is no longer capable of withstanding the crushing pressure of their own gravity. Impossible to reproduce in any laboratory, the fundamental physics of this ultimate material density - its composition and the interactions of its constituents - has remained a mystery since neutron stars were first proposed by Robert Oppenheimer and colleagues in the late 1930s.

NICER has published, to date, robust measurements of the radii of three neutron stars, with mass measurements resulting from a mix of NICER data and independent constraints from radio-telescope observations. Among many other analyses, a peer-reviewed paper by L. Brandes & W. Wolfram (Tech. Univ. of Munich), recently published in the journal Physical Review D, compiles NICER's mass-radius results and applies them to an analysis of the "equation of state" (EOS) of dense matter in the context of bounding information for both lower densities (laboratory measurements of the nuclei of heavy atoms) and much higher densities (the theoretical limits suggested by quantum chromodynamics). An EOS captures the relationship between pressure and density in a given state of matter, which relates directly to the speed of sound in that medium and can be connected to the mass and radius of a self-gravitating star within a given theory of gravity. With generous assumptions for unknown or uncertain quantities, the authors infer a generalized EOS relationship for neutron-star matter as well as some derived insights that are relevant to many astrophysical questions involving neutron stars and black holes: the range of allowed radii for a 1.4 solar-mass neutron star (12.2 ± 0.5 km), the range of radii for a 2.1 solar-mass neutron star (11.9 ± 0.5 km), central densities always less than 5 times that of an atomic nucleus, and the maximum allowed mass that can be supported prior to collapse to a black hole (2.3 solar masses). As NICER's measurements continue to improve, with the accumulation of additional data and the inclusion of other neutron stars, its influence on our understanding of fundamental nuclear physics will deepen further.