Neutron stars are among the most extreme objects in the universe. Formed from the collapse of the 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 cannot be replicated and studied on Earth. That’s why NASA’s mission is to study neutron stars and learn more about the physics that governs the matter that composes them.
My colleagues and I helped them out. 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, giving us the most precise information yet about the strange matter inside it.
What’s inside a neutron star?
The matter in the core of neutron stars is even denser than the nucleus of an atom. As the densest form of stable matter in the universe, it is compressed 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 solve the mysteries of this extreme matter.
NICER is an X-ray telescope on the International Space Station that detects X-rays from hot spots on the surfaces of neutron stars, where temperatures can reach millions of degrees.
Scientists model the timing and energies of these X-rays to map hot spots and determine the mass and size of neutron stars.
Knowing the ratio between the size of neutron stars and their mass allows us to know the “equation of state” of the matter that makes up their core. This allows scientists to know whether the neutron star is soft or hard, and therefore “compressible”, and therefore what it is composed of.
A softer equation of state would suggest that neutrons in the core fragment into an exotic soup of smaller particles. A harder equation of state might mean that neutrons resist, leading to larger neutron stars.
The equation of state also determines how and when neutron stars tear apart when they collide.
Solving the mystery with a nearby neutron star
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 a pulse every time the neutron star rotates.
This particular pulsar spins 173 times per second (as fast as a blender). We’ve been observing it for nearly 30 years with CSIRO’s Murriyang Parkes radio telescope in New South Wales.
The team working with the NICER data faced a challenge with this pulsar. X-rays from a nearby galaxy made it difficult to accurately model hot spots on the neutron star’s surface.
Fortunately, we were able to use radio waves to obtain an independent measurement of the pulsar’s mass. Without this crucial information, the team would not have been able to obtain the exact mass.
Weighing a neutron star is a matter of timing
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 their companion star, a white dwarf—warp space and time. The pulsar and its companion star orbit each other once every 5.74 days.
When the pulsar’s pulses reach us through the compressed space-time surrounding the white dwarf, they are delayed by a few microseconds.
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Such microsecond delays are easily measured with Murriyang from pulsars such as PSR J0437-4715. This pulsar, and other similar millisecond pulsars, are observed regularly 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 Earth orbits 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 and the pulsar.
The mass and size of PSR J0437-4715
We calculated that the mass of this pulsar is typical of a neutron star, 1.42 times the mass of our Sun. This is important because the size of this pulsar should also be that 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 ever found for the equation of state of neutron stars at intermediate densities.
Our new image already allows us to eliminate the softest and hardest equations of state for neutron stars. 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 could include quarks that have escaped from their normal location inside larger particles, or rare particles called hyperons.
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These new data add to an emerging model of neutron star interiors, which has also been informed by observations of gravitational waves from colliding neutron stars and an associated explosion called a kilonova.
Murriyang has a long history of supporting NASA missions and was famous for being the primary receiver of images from most of the Apollo 11 lunar walks.
We have now used this iconic telescope to “weigh in” on the physics of the interiors of neutron stars, advancing our fundamental understanding of the universe.
Daniel Reardon, Postdoctoral Researcher in Pulsar Chronology and Gravitational Waves, Swinburne University of Technology
This article is republished from The Conversation under a Creative Commons license. Read the original article.