NASA's Chandra X-ray Observatory has discovered the first direct evidence for a superfluid, a bizarre, friction-free state of matter, at the core of a neutron star. Superfluids created in laboratories on Earth exhibit remarkable properties, such as the ability to climb upward and escape airtight containers. The finding has important implications for understanding nuclear interactions in matter at the highest known densities.
This composite image shows a beautiful X-ray and optical view of Cassiopeia A (Cas A), a supernova remnant located in our Galaxy about 11,000 light years away. These are the remains of a massive star that exploded about 330 years ago, as measured in Earth's time frame. X-rays from Chandra are shown in red, green and blue along with optical data from Hubble in gold.
Chandra Finds Superfluid in Neutron Star
Credit: X-ray: NASA/CXC/xx; Optical: NASA/STScI, Illustration: NASA/CXC/M.Weiss
At the center of the image is a neutron star, an ultra-dense star created by the supernova. Ten years of observations with Chandra have revealed a 4% decline in the temperature of this neutron star, an unexpectedly rapid cooling. Two new papers by independent research teams show that this cooling is likely caused by a neutron superfluid forming in its central regions, the first direct evidence for this bizarre state of matter in the core of a neutron star.
Neutron stars contain the densest known matter that is directly observable. One teaspoon of neutron star material weighs six billion tons. The pressure in the star's core is so high that most of the charged particles, electrons and protons, merge resulting in a star composed mostly of uncharged particles called neutrons.
Two independent research teams studied the supernova remnant Cassiopeia A, or Cas A for short, the remains of a massive star 11,000 light years away that would have appeared to explode about 330 years ago as observed from Earth. Chandra data found a rapid decline in the temperature of the ultra-dense neutron star that remained after the supernova, showing that it had cooled by about four percent over a 10-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 a team with a paper published in the February 25, 2011 issue of the journal Physical Review Letters. "This means that something unusual is happening within this neutron star."
Superfluids containing charged particles are also superconductors, meaning they act as perfect electrical conductors and never lose energy. The new results strongly suggest that the remaining protons in the star's core are in a superfluid state and, because they carry a charge, also form a superconductor.
"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 a team with a paper accepted in the journal Monthly Notices of the Royal Astronomical Society.
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. The rapid cooling is expected to continue for a few decades and then it should slow down.
"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 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 Celsius. Until now there was a very large uncertainty in estimates of this critical temperature. This new research constrains the critical temperature to between one half a billion to just under a billion degrees.
Cas A will allow researchers to test models of how the strong nuclear force, which binds subatomic particles, behaves in ultradense matter. These results are also important for understanding a range of behavior in neutron stars, including "glitches," neutron star precession and pulsation, magnetar outbursts and the evolution of neutron star magnetic fields.
Small sudden changes in the spin rate of rotating neutron stars, called glitches, have previously given evidence for superfluid neutrons in the crust of a neutron star, where densities are much lower than seen in the core of the star. This latest news from Cas A unveils new information about the ultra-dense inner region of the neutron star.
"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.
The cooling in the Cas A neutron star was first discovered by co-author Craig Heinke, from the University of Alberta, Canada, and Wynn Ho from the University of Southampton, UK, in 2010. It was the first time that astronomers have measured the rate of cooling of a young neutron star.
A University of Alberta astronomer has glimpsed the inner working of a neutron star and found a unique world where the physics can be described as "weird." Craig Heinke's team found the neutron star's core contained a superfluid, a friction-less liquid that could seemingly defy the laws of gravity.
"If you could put some of this superfluid in a jar it would flow up the walls of the container and over the edge," said Heinke.
Heinke says the core of the neutron star also contains a superconductor, a perfect electrical conductor. "An electric current in a superconductor never loses energy—it could keep circulating forever."
These discoveries came about when the researchers used NASA's Chanda space satellite telescope to investigate a sudden temperature drop on one particular neutron star 11,000 light years from Earth. A neutron star is the extremely dense core left behind from an exploding star, or supernova.
Heinke says this neutron star, known as the Cassiopeia A offered the researchers a great opportunity.
"It's only 330 years old," said Heinke. "We've got ringside seats to studying the life cycle of a neutron star from its collapse to its present, cooling off state."
The researchers determined that the neutron star's surface temperature is dropping because its core recently transformed into a superfluid state and is venting off heat in the form of neutrinos, sub atomic particles that flood the universe. Here on Earth our bodies are constantly bombarded by neutrinos, with 100 billion neutrinos passing harmlessly though our eyes every second.
They also found that the neutron star contains a superconductor, the highest temperature (millions of degrees) superconductor known.
This research helps us to better understand the life cycles of stars, as well as the behavior of matter at incredibly high densities.
Page's co-authors were Prakash, James Lattimer (State University of New York at Stony Brook), and Andrew Steiner (Michigan State University.) Shternin's co-authors were Yakovlev, Heinke, Ho, and Daniel Patnaude (Harvard-Smithsonian Center for Astrophysics.)
Contacts and sources:
Chandra X-ray Center
University of Alberta
More information, including images and other multimedia, can be found at:
http://chandra.harvard.edu and http://chandra.nasa.gov