Neutron stars are incredibly dense and compact objects formed from the collapsed cores of supergiant stars. Despite their small size, they weigh more than our Sun. The matter inside neutron stars is in a unique state that cannot be replicated or studied on Earth. NASA is conducting a mission to study neutron stars and learn more about the physics that governs their interiors.
A team of scientists, including myself, has been assisting NASA in this mission. We used radio signals from a fast-spinning neutron star to measure its mass. This measurement allowed scientists working with NASA data to determine the star’s radius, providing us with the most precise information yet about the strange matter inside.
The core of a neutron star is even denser than an atom’s nucleus and is on the verge of collapsing into a black hole. Understanding how matter behaves under these extreme conditions is crucial for testing our theories of fundamental physics.
NASA’s Neutron star Interior Composition ExploreR (NICER) mission aims to unravel the mysteries of this extreme matter. NICER is an X-ray telescope on the International Space Station that detects X-rays emitted from hot spots on the surface of neutron stars. By analyzing the timing and energies of these X-rays, scientists can map the hot spots and determine the mass and size of the neutron stars.
The relationship between the sizes and masses of neutron stars reveals the “equation of state” of the matter in their cores. This equation determines how “squeezeable” the neutron star is and what it is made of. A softer equation of state suggests that neutrons in the core break apart into smaller particles, while a harder equation of state indicates resistance from neutrons, resulting in larger neutron stars. The equation of state also governs how and when neutron stars are torn apart during collisions.
One of NICER’s main targets is a neutron star called PSR J0437-4715, which is the nearest and brightest millisecond pulsar. Pulsars emit beams of radio waves that appear as pulses when observed from Earth. PSR J0437-4715 rotates 173 times per second, and we have been observing it for nearly 30 years using the Parkes radio telescope.
The team faced a challenge in accurately modeling the hot spots on the neutron star’s surface due to X-rays from a nearby galaxy. However, we were able to use radio waves to independently measure the pulsar’s mass, which was crucial for obtaining accurate results.
To measure the mass of the neutron star, we relied on the Shapiro delay, an effect described by Einstein’s theory of general relativity. Massive objects like pulsars warp space and time, causing delays in the arrival of pulses from the pulsar. These microsecond delays can be easily measured with telescopes like Parkes. By observing the wobble in PSR J0437-4715’s orbit caused by Earth’s movement around the Sun, we obtained more information about the orbit’s geometry. Combining this with the Shapiro delay allowed us to determine the masses of the white-dwarf companion and the pulsar.
We calculated that the mass of PSR J0437-4715 is typical for a neutron star, at 1.42 times the mass of our Sun. This also suggests that its size is representative of a typical neutron star. Using NICER data, scientists determined that the neutron star’s radius is 11.4 kilometers, providing a precise anchor point for the neutron star equation of state at intermediate densities.
This new data has ruled out the softest and hardest equations of state for neutron stars and contributes to our understanding of their interiors. The presence of exotic matter, such as escaped quarks or hyperons, in the cores of neutron stars is still being investigated. Observations of gravitational waves from colliding neutron stars and associated explosions have also informed our understanding of neutron star interiors.
The Parkes radio telescope, Murriyang, has a history of assisting NASA missions and was used to receive footage during the Apollo 11 moonwalk. Now, it has played a role in advancing our understanding of the physics of neutron star interiors, contributing to our fundamental knowledge of the universe.
