Neutron star interiors and supernova explosion mechanisms

Partly settled. Some of it is understood, some is still argued about.

Since Ginzburg's problem #24, multi-messenger observations have transformed the field: NASA's NICER mission (from 2019) measured neutron-star masses and radii simultaneously, constraining the equation of state, and the neutron-star merger GW170817 (2017), seen in both gravitational waves and light, further constrained neutron-star matter and confirmed such mergers as a site of heavy-element (r-process) nucleosynthesis. Magnetars have become a well-studied class of neutron star. Nonetheless, the precise equation of state of ultradense matter (hyperons, deconfined quark matter, etc.) and the detailed mechanism that revives a stalled supernova shock remain open theoretical problems.

A neutron star packs more mass than the sun into a sphere the size of a city, and we still do not know exactly what that squeezed matter is made of.

References

• What problems of physics and astrophysics seem now to be especially important and interesting (thirty years later, already on the verge of the XXI century)?, V L Ginzburg, Physics-Uspekhi 42 (4) 353-373, 1999
• A NICER View of PSR J0030+0451: Millisecond Pulsar Parameter Estimation, T. E. Riley et al., The Astrophysical Journal Letters 887, L21, 2019
• Multi-messenger Observations of a Binary Neutron Star Merger, B. P. Abbott et al. (LIGO/Virgo and partner facilities), The Astrophysical Journal Letters 848, L12, 2017