Scientists have found that three neutron starsborn in the fire of other exploding stars, have cooled surprisingly quickly, bringing us closer to understanding the exotic nature of the matter in 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 extinct supernovaand can contain up to almost three times as much mass of our sun in a spherical volume about 6.8 miles (11 kilometers) in diameter. All this matter packed into such a small area means that neutron stars are among the densest concentrations of matter in the known universe, second only to black holes. To make this statement more comparable, consider how a tablespoon of material from a neutron star would be comparable to the mass of Mount Everest.
This extreme nature also means that the physics that govern the interior of neutron stars remains murky. These objects are called neutron stars to begin with because their matter has been crushed to such an extent that it becomes negatively charged. electrons and positively charged protons squish 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 can be crushed to an even greater degree, forming exotic, never-before-seen particles like the hypothetical hyperons. Perhaps, scientists think, or the neutrons themselves could decay into a neutron star, creating a soup of The universethe most fundamental particles: quarks.
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What happens inside a neutron star is governed by the neutron star’s equation of state. Think of it as a game that determines the internal structure and composition of a neutron star based on things like its mass, temperature, magnetic field and so on. The problem is that scientists have literally hundreds of options for what this equation of state could be. Since we cannot replicate to The Earth conditions inside a neutron star, testing which model is correct is highly dependent on matching them with what astronomical observations tell us.
Now, however, the discovery of three neutron stars with significantly lower surface temperatures than other neutron stars of similar age has provided a major clue, allowing researchers to rule out three-quarters of the possible models for the neutron star’s equation of state in one fell swoop. Two of the neutron stars are pulsars, which are rapidly spinning neutron stars that shoot beams of radio jets at us. The third neutron star, in the Vela Jr supernova remnant, does not show pulsar behavior, but this may simply be because its radio jets are not pointing in our direction.
Neutron stars have been detected at X-ray wavelengths of European Space Agency‘c XMM-Newton Telescope and NASA‘c Chandra X-ray Observatory.
“The superior sensitivity of XMM-Newton and Chandra made it possible not only to detect these neutron stars, but also 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 is, the more energetic its X-rays, and the energy of the X-rays from these three neutron stars tells us they’re pretty cool as far as neutron stars go. We say “cool,” but neutron stars are still extremely 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 age, ranging from 840 years to 7,700 years based on the size and expansion rate of the supernova remnants around them, they are considered extremely cold. Neutron stars are born at temperatures of hundreds of billions or even a trillion degrees, and as they cool, other neutron stars of a similar age have temperatures twice that—sometimes even hotter.
Neutron stars can cool by two mechanisms. One is by thermal radiation from their surfaces, allowing heat energy to escape into the cold space. The other is neutrino emission that steals energy from the core of a neutron star and is believed to be responsible for the rapid cooling of this particular trio of neutron stars.
However, how fast neutron stars can cool as a result of these mechanisms depends on the equation of state.
“The young age and cold surface temperature of these three neutron stars can only be explained by triggering a rapid cooling mechanism,” said one of the researchers, Nanda Rea of the Institute of Space Sciences and the Institute of Space Studies of Catalonia, in a statement. “Since the enhanced cooling can only be activated by certain equations of state, this allows us to rule out a significant number of possible models.”
And don’t you just; the team estimates that three-quarters of all possible models can be ignored following this result. The researchers were able to determine this by calculating cool curves, which are basically graphs showing how neutron stars cool with respect to time. The shape of the curve is strongly dependent on neutron star properties such as mass and magnetic field strength, so using machine learning the team calculated the range of parameters that best described each cooling curve and then matched them to the potential equations of the state, seeing which still match and which can be discarded due to zero chance of matching the data.
This process narrowed the range of possible equations of state, but the findings are more than just a characterization of the neutron stars. The behavior of matter at subatomic scales under intense pressure, extreme temperature, and crushing gravity introduces quantum effects too. Scientists currently lack a the quantum theory of gravityand the neutron star equation of state could therefore lead us to achieve quantum effects and high-gravity finally physics together as one theory.
The findings are described in a paper published June 20 in the journal Nature Astronomy.
