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?
In 2024 we made our tightest measurement of radius to date, for a pulsar with a mid-range mass of 1.4 solar masses. This measurement indicated a "softer" equation of state than previously thought. This means that the maximum mass of neutron stars must be lower than some theories predict. And this, in turn, fits nicely with what observations of gravitational waves seem to suggest. And the magnetic field is again very far from a simple dipole!
As this project comes to a close we now have measurements of mass and radius for 5 NICER pulsars, and have started to extend the technique to other types of neutron star. By making surface maps, and measuring their sizes, we can shed light on both their interiors and their fascinating surfaces, using relativity as our tool of choice.
