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The physics and forensics of neutron star explosions

Periodic Reporting for period 4 - CSINEUTRONSTAR (The physics and forensics of neutron star explosions)

Reporting period: 2019-12-01 to 2020-05-31

Densities in neutron stars can reach up to ten times the density of a normal atomic nucleus. Under these conditions, we anticipate the formation of unusual states of matter including stable states of strange matter that cannot be formed in colliders on Earth. In order to figure out the nature of the dense matter within neutron stars, we need to be able to measure their mass and radius very precisely. This is difficult because they are so small (only 10-15 km in radius) and so very far away. One very promising technique for simultaneous measurement of mass and radius uses anomalously bright hotspots that form on the neutron star surface when plasma accreted from a companion star undergoes a thermonuclear explosion. As the star spins and hotspot rotates in and out of our line of sight, the brightness of the star varies. The strong gravitational field and relativistic rotation speeds of the surface encode information about the star's mass and radius in this variation. However the mechanism that generates these hotspots remains unknown, 20 years after their discovery. This introduces uncertainty into our attempts to measure mass and radius. Thermonuclear ignition conditions, flame spread, and the magnetohydrodynamics of the accreted neutron star ocean all play a role. The goal of the CSINEUTRONSTAR project was to solve the mechanism that causes the hotspots to develop, and to develop the technique of using such hotspots to measure neutron star mass and radius. Although we have yet to definitively identify the mechanism, we have identified several promising mechanisms including ocean waves and convective patterns - with relativity, nuclear burning, and the rapid spin of the stars all playing an important role. Using data from NASA's NICER telescope on the International Space Station, were also succeeded in delivering the first measurement of a neutron star's mass and radius using the hotspot technique. Our analysis also revealed that the magnetic field of the pulsar is very far from the simple bar magnet field structure depicted on the NASA mission patch.
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.
The project has delivered the first results from the Pulse Profile Modelling technique for neutron star mass-radius inference (our group was one of two independent teams within the NICER collaboration carrying out the inference analysis).

It has also delivered viable models for the burst oscillation mechanism. Further investigation will be required to pin this down, but we have identified the important physical processes that need to be included in the modelling.