Neutron stars offer a unique environment in which to develop and test theories of the strong force. Densities in neutron star 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 matter that is neutron-rich, and opens up the possibility of stable states of strange matter, something that can only exist in neutron stars. Strong force physics is encoded in the Equation of State (EOS), the pressure-density relation, which links to macroscopic observables such as mass M and radius R via the stellar structure equations. By measuring and inverting the M-R relation we can recover the EOS and diagnose the underlying dense matter physics.
One very promising technique for simultaneous measurement of M and R exploits hotspots (burst oscillations) that form on the neutron star surface when material accreted from a companion star undergoes a thermonuclear explosion (a Type I X-ray burst). As the star rotates, the hotspot gives rise to a pulsation. Relativistic effects then encode information about M and R into the pulse profile. However the mechanism that generates burst oscillations remains unknown, 20 years after their discovery. Ignition conditions, flame spread, and the magnetohydrodynamics of the star's ocean all play a role. I will review the progress that we are making towards cracking this long-standing problem, and establishing burst oscillations as a tool par excellence for measuring M and R. This is a major goal for future large area X-ray telescopes such as eXTP and STROBE-X.
