Part of our research involves studying how hotspots might form on the oceans of neutron stars as they explode in thermonuclear bursts. We focused on two particular aspects: the development of large-scale wave patterns in the ocean, and the role of convection. Large-scale wave patterns had been studied id the past and discounted due to problems in reconciling theory and observation: our new models, incorporating the latest understanding of how the ocean burns, and relativistic effects, resolve some of these issues. We have also shown that convection, driven by the explosion, can give rise to interesting patterns, which may form the seed for hotspot formation.
The other part of the project looked at how to use hotspots to determine the nature of the ultradense matter in the cores of neutron stars, using a relativistic ray-tracing based inference technique. We developed a highly-efficient simulation and inference code to do this, and worked closely with nuclear physics colleagues to develop the methodology necessary to map the measured masses and radii to nuclear physics parameters. Using these capabilities we used data from NASA's Neutron Star Interior Composition Explorer (NICER) telescope - installed on the International Space Station in 2017 - to deliver the first mass-radius measurements obtained with this new technique. These results are already being used, in combination with constraints from gravitational wave data, to constrain the properties of ultradense nuclear matter. As a by-product, the technique also maps the locations of the magnetic polar caps on the star - to our surprise these indicated that the field was very far from the simple dipole magnetic field often assumed in pulsar studies, posing new challenges for theorists.
We have published our results in just over 30 refereed journal papers, including an Astrophysical Journal Letters Focus Issue on the NICER results.
