Periodic Reporting for period 2 - AEONS (Advancing the Equation of state of Neutron Stars)
Reporting period: 2021-12-01 to 2023-05-31
Neutron stars, the collapsed cores of stars more massive than our Sun, are fascinating objects. They contain as much matter as the Sun, crushed into a sphere that is only 20-30 km in diameter, the size of a city. Densities inside neutron stars can reach several times that of a normal atomic nucleus. This makes them unique places to study dense matter nuclear physics – if stable states of strange quark matter exist anywhere in the Universe it will be in the cores of neutron stars.
To figure out what lies inside neutron stars, astronomers need to measure their mass and size. Neutron stars have both extremely strong gravity (due to their high density) and rapid rotation (they can spin up to several hundred times a second!). Relativity imprints information about the star’s mass and radius on any radiation emitted from the neutron star surface. By measuring and modelling this, we can extract mass and radius.
NASA’s NICER, a telescope on the International Space Station, is now letting us pioneer a new way of doing this, for neutron stars with bright X-ray emitting hotspots at their magnetic poles. Pulse Profile Modelling (PPM) delivers not only mass and radius but also – as a byproduct – surface maps of the hotspots on these tiny stars thousands of lightyears from Earth. For the NICER sources this relates to the magnetic field structure of the star, helping us to resolve basic questions about stellar evolution.
One goal of this ERC project is to deliver NICER’s PPM analysis, including carrying out a major programme of simulation and testing to ensure that we can have full confidence in the results delivered by this NASA mission. And we are looking at how to apply the technique to different types of neutron stars with hotspots that we will be able to study in exquisite detail with the next generation of space telescopes. Our results are improving our understanding of both the nature of matter and the fundamental properties of neutron stars.
To figure out what lies inside neutron stars, astronomers need to measure their mass and size. Neutron stars have both extremely strong gravity (due to their high density) and rapid rotation (they can spin up to several hundred times a second!). Relativity imprints information about the star’s mass and radius on any radiation emitted from the neutron star surface. By measuring and modelling this, we can extract mass and radius.
NASA’s NICER, a telescope on the International Space Station, is now letting us pioneer a new way of doing this, for neutron stars with bright X-ray emitting hotspots at their magnetic poles. Pulse Profile Modelling (PPM) delivers not only mass and radius but also – as a byproduct – surface maps of the hotspots on these tiny stars thousands of lightyears from Earth. For the NICER sources this relates to the magnetic field structure of the star, helping us to resolve basic questions about stellar evolution.
One goal of this ERC project is to deliver NICER’s PPM analysis, including carrying out a major programme of simulation and testing to ensure that we can have full confidence in the results delivered by this NASA mission. And we are looking at how to apply the technique to different types of neutron stars with hotspots that we will be able to study in exquisite detail with the next generation of space telescopes. Our results are improving our understanding of both the nature of matter and the fundamental properties of neutron stars.
In 2021 we reported the size of the heaviest known pulsar, PSR J0740+6620. Despite the star being 50% more massive than a star the NICER team had measured in 2019, the heavier neutron star was essentially the same size – about 26 km in diameter. This challenges some of the more squeezable models of neutron star cores, including versions where the interior is just a sea of quarks. PSR J0740+6620’s size and mass also pose problems for some less squeezable models containing only neutrons and protons.
Since then we have continued to refine this measurement with new data, showing definitively that the magnetic field is not a simple symmetric dipole, but something far more complex (see graphic). Understanding how this came to be is something that we are also working hard on – has it been like this since the neutron star’s birth in a supernova, or did it evolve to this in a later phase of the neutron star’s life?
Since then we have continued to refine this measurement with new data, showing definitively that the magnetic field is not a simple symmetric dipole, but something far more complex (see graphic). Understanding how this came to be is something that we are also working hard on – has it been like this since the neutron star’s birth in a supernova, or did it evolve to this in a later phase of the neutron star’s life?
The precision measurements that we are delivering with NICER are the first of their kind, providing unique insight into the nature of dense matter and the surface physics of neutron stars. They are supported by a rigorous programme of simulation and testing, and the next few years will see several new measurements being delivered by the NICER team, placing new constraints on ultradense nuclear matter.
We are also making excellent progress on our goal of extending the PPM technique to different types of neutron star that have hotspots. Accreting neutron stars – where hotspots form via magnetically-channeled accretion from a companion star, or from thermonuclear explosions in the accreted ocean layers – pose new challenges for PPM, such as motion of the hotspots. This requires both new physics in our models and more advanced computational techniques, which this project will deliver.
We are also making excellent progress on our goal of extending the PPM technique to different types of neutron star that have hotspots. Accreting neutron stars – where hotspots form via magnetically-channeled accretion from a companion star, or from thermonuclear explosions in the accreted ocean layers – pose new challenges for PPM, such as motion of the hotspots. This requires both new physics in our models and more advanced computational techniques, which this project will deliver.