Neutron stars are some of 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 compressed matter in a unique state that we cannot replicate and study on Earth. That’s why NASA is on a mission to study neutron stars and learn about the physics that governs the matter inside them.
My colleagues and I have helped them. We used radio signals from a rapidly rotating 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 is inside a neutron star?
The matter in the core of neutron stars is even denser than the nucleus of an atom. As the densest stable form of matter in the universe, it has been pushed to its limit and on the verge of collapsing into a black hole.
Understanding how matter behaves under these conditions is a key test of our theories of fundamental physics.
NASA’s Neutron Star Interior Composition ExploreR (NICER) mission is trying to unravel the mysteries of this extreme matter.
NICER is an X-ray telescope on the International Space Station. It detects X-rays coming 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 hotspots and determine the mass and size of neutron stars.
Knowing how the sizes of neutron stars relate to their masses will reveal 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 milder equation of state would suggest that the neutrons in the nucleus are splitting into an exotic soup of smaller particles. A stronger equation of state could mean that neutrons resist, leading to larger neutron stars.
The equation of state also dictates how and when neutron stars break apart when they collide.
Solving the mystery with a neutron star neighbor
One of NICER’s main targets is a neutron star called PSR J0437-4715, which is the closest and brightest millisecond pulsar.
A pulsar is a neutron star that emits beams of radio waves that we observe as pulses whenever the neutron star rotates.
This particular pulsar spins 173 times per second (as fast as a blender). We’ve been observing it for almost 30 years with Murriyang, CSIRO’s Parkes radio telescope in New South Wales.
The team working with the NICER data faced a challenge for this pulsar. X-rays coming from a nearby galaxy made it difficult to accurately model hotspots 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 important information, the team would not have recovered the correct measure.
The weighting of a neutron star is related to time
To measure the mass of the neutron star, we rely on an effect described by Einstein’s theory of general relativity, called the Shapiro delay.
Massive, dense objects like pulsars—and in this case its companion star, a white dwarf—distort space and time. The pulsar and this companion rotate once every 5.74 days.
When pulses from the pulsar travel to us through the compressed space surrounding the white dwarf, they are delayed by microseconds.
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Such microsecond delays are easy to measure with 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 oscillation gives us more details about the geometry of the orbit. We use this together with the Shapiro delay to find the masses of the white dwarf companion and the pulsar.
Mass and size of PSR J0437-4715
We calculated that the mass of this pulsar is typical of a neutron star, with 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 precise anchor point yet found for the neutron star equation of state at intermediate densities.
Our new picture already rules out the softer and harder equations of state for the neutron star. Scientists will continue to decipher exactly what this means for the presence of exotic matter in the inner cores of neutron stars.
Theories suggest that this matter may involve quarks that have escaped from their normal homes inside larger particles, or rare particles known as hyperons.
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These new data add to a developing model of neutron star interiors, which has also been informed by observations of gravitational waves from colliding neutron stars and an accompanying explosion called a kilonova.
Murriyang has a long history of assisting NASA missions and was used extensively as the main image receiver for much of the Apollo 11 moonwalk.
Now, we’ve used this iconic telescope to “weigh in” on the inner physics of neutron stars, advancing 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.