How neutron stars 'playing it cool' could unlock exotic physics

Scientists have found that three neutron stars, born in the fires of other exploding stars, have cooled off surprisingly quickly, bringing us closer to understanding the exotic nature of matter within the cores of these extreme objects.

The discovery was made by a Spanish team led by Alessio Marino of the Institute of Space Sciences (ICE–CSIC) in Barcelona, using European and American space telescopes that work with X-ray light.

A neutron star is the collapsed core of a massive star that has gone supernova, and can contain up to nearly three times the mass of our sun in a spherical volume just about 6.8 miles (11 kilometers) across. All that matter compacted into such a small area means neutron stars are among the densest concentrations of matter in the known universe, second only to black holes. To make that statement more relatable, consider how a tablespoon of neutron-star material would be comparable to the mass of Mount Everest.

This extreme nature  also means the physics that governs neutron stars' interiors remains murky. These objects are called neutron stars to begin with because their matter has been crushed to such a degree that negatively charged electrons and positively charged protons get smushed together, overcoming the electrostatic force between them to form an object full of just neutral neutrons. Deeper in the core of a neutron star, matter may be crushed to an even greater extent, forming exotic, never-before-seen particles such as hypothetical hyperons. Perhaps, scientists believe, or neutrons themselves could be popped apart within a neutron star, creating a soup of the universe's most fundamental particles: quarks.

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What happens inside a neutron star is governed by the neutron star equation of state. Think of this as a playbook that determines a neutron star's internal structure and composition based on things like its mass, temperature, magnetic field and so on. The trouble is, scientists have literally hundreds of options for what this equation of state could be. Since we cannot replicate on Earth the conditions inside a neutron star, testing which model is the right one is highly dependent on matching them to what astronomical observations tell us.

Now, however, the discovery of three neutron stars with substantially lower surface temperatures compared to other neutron stars of similar age has provided a big clue, allowing researchers to rule out three-quarters of the possible models for the neutron star equation of state in one stroke. Two of the neutron stars are pulsars, which are rapidly spinning neutron stars that fire beams of radio jets toward us. The third neutron star, in the Vela Jr supernova remnant, doesn't display pulsar behavior, but that may just be because its radio jets do not point in our direction.

The neutron stars were detected at X-ray wavelengths by the European Space Agency's XMM-Newton telescope and NASA's Chandra X-ray Observatory. 

"The superb sensitivity of XMM-Newton and Chandra made it possible not only to detect these neutron stars, but to collect enough light to determine their temperatures and other properties," said Camille Diez, who is an XMM-Newton scientist at the European Space Agency, in a statement.

The hotter a neutron star, the more energetic its X-rays, and the energy of the X-rays from these three neutron stars tells us that they are pretty chilly as far as neutron stars go. We say "chilly," but the neutron stars are still exceptionally hot, with temperatures ranging from 1.9 million to 4.6 million degrees Celsius (3.4 million to 8.3 million degrees Fahrenheit). However, for their young ages, ranging from 840 years to 7,700 years based on the size and expansion velocity of the supernova remnants around them, they are considered outstandingly cold. Neutron stars are born with temperatures of hundreds of billions, or even a trillion, degrees, and while they do cool, other neutron stars of similar ages have temperatures twice as high — sometimes even hotter than that. 

Neutron stars can cool via two mechanisms. One is through thermal radiation from their surfaces that allows heat energy to escape into the cold of space. The other is neutrino emission, which steals away energy from the core of a neutron star, and is thought to be responsible for the fast cooling of this particular neutron star trio.

However, how fast neutron stars can cool as a result of these mechanisms depends on the equation of state. 

"The young age and the cold surface temperature of these three neutron stars can only be explained by invoking a fast cooling mechanism," one of the researchers, Nanda Rea of the Institute of Space Sciences and Institute of Space Studies of Catalonia, said in the statement. "Since enhanced cooling can be activated only by certain equations of state, this allows us to exclude a significant portion of the possible models."

And didn't they just; the team estimates that three-quarters of all possible models can be disregarded after this result. The researchers were able to determine this by calculating cool curves, which are basically graphs that show how neutron stars cool with respect to time. The shape of the curve is highly dependent on properties of the neutron stars such as mass and magnetic-field strength, so, by using machine learning, the team calculated the range of parameters that best describe each cooling curve, and then matched these to potential equations of state, seeing which ones still matched and which ones could be thrown out for having zero chance of matching the data.

This process has narrowed down the range of possible equations of state, but the findings are about more than just characterizing neutron stars. The behavior of matter on subatomic scales under intense pressure, extreme temperature and crushing gravity introduces quantum effects, too. Scientists currently lack a quantum theory of gravity, and an equation of state for neutron stars could therefore set us on the road to bringing quantum effects and high-gravity physics together as a single theory at last.

The findings are described in a paper published on June 20 in the journal Nature Astronomy.