Densities in neutron star (NS) cores can reach up to ten times the density of a normal atomic nucleus, and the stabilising effect of gravitational confinement permits long-timescale weak interactions. This generates nucleonic matter that is extremely neutron-rich, and the exciting possibility of stable states of strange matter (hyperons or deconfined quarks). Our uncertainty about the nature of cold ultradense matter is encoded in the Equation of State (EOS), which can be mapped via the stellar structure equations to quantities like mass M and radius R that determine the exterior space-time.
One very promising technique for measuring the EOS exploits hotspots that form on the NS surface due to the pulsar mechanism, accretion streams, or during thermonuclear explosions in the stellar ocean. As the NS rotates, the hotspot gives rise to a pulsation and relativistic effects encode information about the EOS into the pulse profile. Pulse Profile Modelling (PPM), which employs relativistic ray-tracing and Bayesian inference codes to measure M-R and the EOS, is being pioneered by NASA’s NICER telescope, which is poised to deliver its first results in 2019.
Complexities, that have only become apparent with exposure to real data, mean that there is work to be done if we are to have confidence in the nominal 5-10% accuracy of NICER’s M-R results. AEONS will deliver this. The project will also look ahead to the next generation of large-area X-ray timing telescopes, since it is only then that PPM will place tight constraints on dense matter models. The sources these missions target, accreting neutron stars, pose challenges for PPM such as variability, surface pattern uncertainty, and polarimetric signatures. AEONS will develop a robust pipeline for accreting NS PPM and embed it in a multi-messenger EOS inference framework with radio and gravitational wave constraints. This will ensure that PPM delivers major advances in our understanding of the nature of matter.
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