Neutron stars are among the most extreme objects in the universe. Formed from the collapsed cores of supergiant stars, they weigh more than our sun and yet are compressed into a sphere the size of a city.
The dense cores of these exotic stars contain matter compressed into unique states that are impossible for us to replicate and study on Earth. That’s why NASA is on a mission to study neutron stars and learn more about the physics that governs the matter inside them.
My colleagues and I helped them. We used radio signals from a rapidly spinning neutron star to measure its mass. This allowed scientists working with NASA data to measure the star’s radius, which in turn gave us the most precise information yet about the strange matter inside.
What’s inside a neutron star?
Matter in the core of neutron stars is even denser than the core of an atom. As the densest stable form of matter in the universe, it is being squeezed to the limit and is on the verge of collapsing into a black hole.
Understanding how matter behaves under these conditions is an important test of our theories of fundamental physics.
NASA’s Neutron star Interior Composition ExploreR (NICER) mission aims to solve the mysteries of this extreme matter.
NICER is an X-ray telescope on the International Space Station. It detects X-rays from hot spots on the surface of neutron stars where temperatures can reach millions of degrees.
Scientists model the timing and energies of these X-rays to map the hot spots and determine the masses and sizes of the neutron stars.
Knowing the size of neutron stars relative to their masses reveals the “equation of state” of the matter in their cores. This tells scientists how soft or hard – how “squeezable” – the neutron star is, and therefore what it is made of.
A softer equation of state would suggest that neutrons decay in the core into an exotic soup of smaller particles. A harder equation of state could imply that neutrons resist, leading to larger neutron stars.
The equation of state also determines how and when neutron stars are torn apart when they collide.
Solving the mystery with a neutron star as a neighbor
One of NICER’s main targets is a neutron star called PSR J0437-4715. This is the closest and brightest millisecond pulsar.
A pulsar is a neutron star that emits radio waves. We see these waves as a pulse every time the neutron star rotates.
This particular pulsar spins 173 times per second (as fast as a blender). We have been observing it for almost 30 years using Murriyang, CSIRO’s Parkes radio telescope in New South Wales.
The team working with NICER data had a challenge for this pulsar. X-rays from a nearby galaxy made it difficult to accurately model the hot spots on the neutron star’s surface.
Fortunately, we were able to use radio waves to find an independent measurement of the pulsar’s mass. Without this crucial information, the team would not have been able to determine the correct mass.
Weighing a neutron star is all about timing
To measure the mass of the neutron star, we use an effect described in Einstein’s general theory of relativity: the Shapiro delay.
Massive and dense objects such as pulsars – and in this case their associated star, a white dwarf – curve space and time. The pulsar and its associated star orbit each other once every 5.74 days.
When pulses from the pulsar travel to us through the compressed space-time around the white dwarf, they are delayed by microseconds.
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Such microsecond delays are easy to measure using Murriyang from pulsars such as PSR J0437-4715. This pulsar, and other millisecond pulsars like it, are regularly observed by the Parkes Pulsar Timing Array project, which uses these pulsars to detect gravitational waves.
Because PSR J0437-4715 is relatively close to us, its orbit appears to wobble slightly from our perspective as the Earth moves around the Sun. This wobble gives us more details about the geometry of the orbit. We use this, along with the Shapiro delay, to find the masses of the white dwarf companion and the pulsar.
The mass and size of PSR J0437-4715
We calculated that the mass of this pulsar is typical for a neutron star, namely 1.42 times the mass of our sun. This is important because the size of this pulsar should also be the size of a typical neutron star.
Scientists working with the NICER data were then able to determine the geometry of the X-ray hotspots and calculate that the radius of the neutron star is 11.4 kilometers. These results provide the most accurate anchor point yet for the neutron star equation of state at intermediate densities.
Our new picture already rules out the softest and hardest equations of state for neutron stars. Scientists will continue to decode what exactly this means for the presence of exotic matter in the inner cores of neutron stars.
Theories suggest that this matter may contain quarks that have escaped from their normal homes in larger particles, or rare particles called hyperons.
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These new data complement a new model of the interior of neutron stars, which is also based on observations of gravitational waves from colliding neutron stars and an associated explosion, called a kilonova.
Murriyang has a long history of assisting NASA missions, including being the primary recipient of images from most of Apollo 11’s moonwalk.
Now we have used this iconic telescope to give our verdict on the physics of the interiors of neutron stars, expanding our fundamental understanding of the universe.
Daniel Reardon, Postdoctoral Researcher in Pulsar Timing and Gravitational Waves, Swinburne University of Technology
This article is republished from The Conversation under a Creative Commons license. Read the original article.