The discovery of the rapid decline in the temperature of an ultradense star has provided the first evidence for a bizarre state of matter in the core of a star. Two independent research teams have used data from NASA's Chandra X-ray Observatory to show that the interior of a neutron star contains superfluid and superconducting matter, a conclusion with important implications for understanding nuclear interactions in matter at the highest known densities. The teams publish their research separately in the journals Monthly Notices of the Royal Astronomical Society Letters and Physical Review Letters.
This image presents a beautiful composite of X-rays from Chandra (red, green, and blue) and optical data from Hubble (gold) of Cassiopeia A, the remains of a massive star that exploded in a supernova. Evidence for a bizarre state of matter has been found in the dense core of the star left behind, a so-called neutron star, based on cooling observed over a decade of Chandra observations. The artist’s illustration in the inset shows a cut-out of the interior of the neutron star where densities increase from the crust (orange) to the core (red) and finally to the region where the “superfluid” exists (inner red ball). Credit: X-ray: NASA/CXC/UNAM/Ioffe/D.Page,P.Shternin et al; Optical: NASA/STScI; Illustration: NASA/CXC/M.Weiss
This news comes from studies of the supernova remnant Cassiopeia A (Cas A), the remains of a massive star that exploded about 330 years ago in Earth's time-frame. A sequence of Chandra observations of the neutron star, the ultra-dense core that remained after the supernova, shows that this compact object has cooled by about 4% over a ten-year period.
"This drop in temperature, although it sounds small, was really dramatic and surprising to see," said Dany Page of the National Autonomous University in Mexico, leader of one of the two teams. "This means that something unusual is happening within this neutron star."
Neutron stars contain the densest known matter that is directly observable. One teaspoon of neutron star material has a mass of six billion tons. The pressure in the star's core is high enough that most of the electrons there are forced to merge with protons, producing neutrons. This leaves a star composed mostly of neutrons, with some protons, electrons and other particles.
Theoretical physicists have come up with detailed models for how matter should behave at such high densities, including the possibility that superfluids may form. Superfluidity is a friction-free state of matter, and superfluids created in laboratories on Earth exhibit remarkable properties, such as the ability to climb upward and escape airtight containers. Superfluids made of charged particles are also superconductors, which allow electric current to flow with no resistance. (Materials like this on Earth have widespread technological applications like producing the superconducting magnets used for the magnetic resonance imaging (MRI) machines found in hospitals).
"The rapid cooling in Cas A's neutron star, seen with Chandra, is the first direct evidence that the cores of these neutron stars are, in fact, made of superfluid and superconducting material," said Peter Shternin, of the Ioffe Institute in St Petersburg, Russia, leader of the second team.
Both teams show that this rapid cooling is explained by the formation of a neutron superfluid in the core of the neutron star, within about the last 100 years as seen from Earth. Theory predicts a neutron star should undergo a distinct cool-down during the transition to the superfluid state as nearly massless, weakly interacting particles, called neutrinos, are copiously formed and then escape from the star, taking energy with them. The rapid cooling is expected to continue for a few decades and then it should slow down.
The onset of superfluidity in materials on Earth occurs at extremely low temperatures near absolute zero, but in neutron stars, it can occur at temperatures near a billion degrees because interactions of particles are via the strong nuclear force (this force binds quarks together to make protons and neutrons, and protons and neutrons together to make the nuclei of atoms). However, until now there was a very large uncertainty in estimates of this critical temperature. This new research constrains it to between half a billion and just under a billion degrees Celsius.
"It turns out that Cas A may be a gift from the Universe because we would have to catch a very young neutron star at just the right point in time," said Page's co-author Madappa Prakash, from Ohio University. "Sometimes a little good fortune can go a long way in science."
The observed rate of cooling strongly suggests that the relatively few remaining protons in the core of the neutron star made the transition to the superfluid state much earlier. Because the protons are charged they will also be superconducting.
"Previously we had no idea how extended superconductivity of protons was in a neutron star," said Shternin's co-author Dmitry Yakovlev, also from the Ioffe Institute in St Petersburg, Russia.
Team member Wynn Ho, of the University of Southampton in the UK, adds, "Depending on their composition, superconductors created in laboratories on Earth stop working at anything warmer than -100 to -200 degrees Celsius. In contrast the incredible densities in neutron stars allow superconductivity at close to a billion degrees Celsius."
Since models for superfluidity in neutron stars incorporate the physics of the strong nuclear force, the detailed features of the strong interaction in ultradense matter can be tested in the Cas A neutron star. These results are also important for understanding a range of behaviour in neutron stars, including glitches (these are small sudden changes in highly magnetized rotating neutron stars, objects known as pulsars), neutron star precession and pulsation, magnetar outbursts and the evolution of neutron star magnetic fields.
Glitches have previously given evidence for superfluid neutrons in the crust of a neutron star where densities are below the nuclear values seen in the core of the star. The research on Cas A provides the first direct evidence for superfluid neutrons and protons in the core of a neutron star.
The cooling in the Cas A neutron star was first discovered by co-authors Craig Heinke, from the University of Alberta, Canada, and Wynn Ho from the University of Southampton in 2010. It was the first time that astronomers have measured the rate of cooling of a young neutron star.
Dany Page's team recently had their paper accepted in Physical Review Letters. His co-authors were Madappa Prakash, James Lattimer from the State University of New York at Stony Brook and Andrew Steiner, from Michigan State University. Peter Shternin's team submitted their paper only two days after the Page team and it was recently accepted in the journal Monthly Notices of the Royal Astronomical Society: letters. His co-authors were Dmitry Yakovlev, Craig Heinke, Wynn Ho and Daniel Patnaude from the Harvard-Smithsonian Center for Astrophysics.
Wynn Ho's work was partly funded by the UK's Science and Technology Facilities Council.
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IMAGES AND CAPTIONS
Images available from http://chandra.harvard.edu/photo/2011/casa/ and http://www.ras.org.uk/
Chandra on Cas A
MNRAS letters preprint
Phys. Rev. Lett. preprint