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The physics and forensics of neutron star explosions

Periodic Reporting for period 3 - CSINEUTRONSTAR (The physics and forensics of neutron star explosions)

Reporting period: 2018-06-01 to 2019-11-30

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 is to solve the mechanism that causes the hotspots to develop, and then use them to measure neutron star mass and radius. This is a major goal of the next generation of large X-ray space telescopes.
Part of our research involves studying how hotspots might form on the oceans of neutron stars as they explode in thermonuclear bursts. We have 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 in the past and discounted due to problems in reconciling theory and observation: our new models, incorporating the latest understanding of how the ocean burns, resolve some of these issues. We have also shown that convection can give rise to interesting patterns, which may form the seed for hotspot formation.

We have also been studying 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. Work to date has involved development of codes necessary to assess this in cases when the surface pattern is evolving, as it is during thermonuclear bursts . We are now deploying these codes to determine the best constraints that can be extracted from current data and to set the science requirements and analysis techniques for the next generation of large area X-ray telescopes (which will exploit this technique).
During the next phase of the project we will analyse in more detail the viability of ocean wave patterns, taking into account the full variation from new nuclear physics models, and the zonal flows induced by convection. We will deliver the best constraints on the dense matter equation of state that can be delivered from archival X-ray data, and set out the analysis methodology to be used for the next generation of X-ray space telescopes.