Superfluids and superconductors in the core of a neutron star: the highest-temperature superconductor

This page gives some information on our discovery of evidence for superfluids and superconductors in neutron star cores, based on the rapid cooling of a young neutron star. Press releases and links to published stories will be posted at the bottom. The papers are Page et al. and Shternin et al., which explain how the observations require superfluids and superconductors, and Heinke & Ho (2010), which showed that the neutron star is cooling. Please email me (heinke (at) ualberta [dot] ca) or my colleagues (full list at bottom) with any questions.

The Chandra X-ray Observatory (NASA).

We used the Chandra X-ray Observatory to study X-rays coming from the neutron star. X-rays are electromagnetic radiation, just like visible light, except that X-rays are higher energy (and thus shorter wavelength). X-rays are blocked by the Earth's atmosphere, so to observe them we use satellites like NASA's Chandra telescope. Neutron stars are tiny, incredibly dense remnants of dead stars, compressing as much mass as our Sun into a star the size of a city.

Astronomers have observed the remains of numerous supernovae, or exploding stars, using optical telescopes, as well as X-ray and radio telescopes. Supernovae are the deaths of massive stars, more than 8 times more massive than our sun. During the supernova, the core (about 1.4 times the mass of our sun) collapses into a tiny stellar remanant, while the rest of the star is blown out into space (producing a beautiful nebula). The core, a neutron star, is compressed to densities higher than atomic nuclei, made up mostly of neutrons and protons jam-packed together. These high densities cause matter to behave in strange ways that can't be studied on Earth.

Deep Chandra X-ray image of the young Cassiopeia A supernova remnant. Credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.

The Cassiopeia A (or Cas A) supernova remnant contains the youngest known neutron star. Chandra identified the neutron star inside the Cas A remnant, and has observed this neutron star repeatedly over the last ten years. This neutron star's surface temperature is so hot (several million degrees) that it emits X-rays. In 2009, Wynn Ho and I figured out how to interpret the X-ray emission from this neutron star (see my Cas A page and references therein for details.)

A combined Chandra X-ray and Hubble optical image of the supernova remnant Cassiopeia A, with an artist's impression of the neutron star at the center of the remnant, with its (invisible) neutrino wind. Credit: Chandra image and illustration: NASA/CXC/D. Page/P. Shternin

Last year, Wynn Ho and I discovered that the neutron star was visibly cooling down over time--decreasing in temperature by 4% over 10 years. We published this result, and now two teams of theorists (led by Dany Page, of UNAM, Mexico City, Mexico, and Peter Shternin, of Ioffe Technical Institute, St. Petersburg, Russia--I'm collaborating with the Shternin group) have figured out what this temperature drop means. The cooling is too fast to be explained by the X-ray radiation from its surface, but can be explained if the neutron star is emitting neutrinos (tiny subatomic particles that pass right through matter) from its core.

There are several different ways that neutron stars could emit neutrinos, and theorists haven't been sure which methods operate. Drs. Page and Shternin, looking at the temperature measurements, each concluded that the cooling must be caused by the neutrons and protons being superfluids, the protons thus being superconducting. That's a lot to understand--let's take it in pieces.

Superfluids are an unusual state of matter, where the fluid experiences no friction--a strange effect of quantum mechanics. Superfluids climb up and out of unsealed containers (as shown above for liquid helium), rotate by forming vortices, and have other strange properties. They are formed when their particles pair up; their quantum spin states become aligned. Since the pairing is very delicate, it can be destroyed by high temperatures. On earth, superfluids are found when some materials (like liquid helium) are cooled to a few degrees above absolute zero.

Superconductors are perfect electrical conductors, making them extremely useful, as extremely strong magnets can be created. Applications include medical imaging devices (MRI, NMR), particle accelerators (e.g. the LHC), and magnetically levitating trains (in Shanghai, China). A famous property of superconductors is their ability to repel magnetic fields, thus forcing a magnet to levitate (e.g. photo above, by David Monnia). Superconductors are similar to superfluids--they are formed by the pairing of charged particles. On Earth, superconductors involve paired electrons, and like superfluids, the superconducting effects go away at high temperatures, requiring that superconductors be cooled by (e.g.) liquid nitrogen to temperatures like -135 Celsius.

Neutron stars have long been suspected (e.g. Migdal 1959) to contain superfluid neutrons, and superfluid (thus, since protons are charged, superconducting) protons. There is evidence (Anderson & Itoh 1975) that neutron stars contain superfluid neutrons in their outer layers (their so-called "crusts"). Such superfluids and superconductors can exist at much higher temperatures inside neutron stars than on Earth, because they involve different particles and extremely high densities. However, until now there has not been direct evidence for neutron superfluidity and proton superconductivity in the cores of neutron stars.

When two neutrons pair up, they fall into a lower-energy state. The extra energy is released as neutrinos, which easily escape from the neutron star into space. Thus the pairing of neutrons rapidly cools the neutron star. Neutron pairs may be broken (by being "bumped" by other neutrons), and re-form; every time a pair forms, neutrinos are emitted. This cooling by pair formation can only happen when the neutron star interior is cool enough to become a superfluid. The Page and Shternin groups are able to explain the rapid neutron star cooling by saying that the neutrons have only recently become superfluid--giving a superfluid transition temperature of 0.5-1 billion degrees K. They also need proton superconductivity to exist in the neutron star, in order to suppress other cooling mechanisms until neutron pair formation starts the rapid cooling.

So to summarize; we've finally provided strong, direct evidence that neutron stars have neutron superfluids and proton superconductors in their cores. This makes neutron stars the highest-temperature superconductors known (by far!). It gives us insight into how matter behaves at high densities, and into how neutron stars evolve over time.

My collaborators are Wynn Ho (wynnho (at) slac [dot] stanford [dot] edu) of Southampton Univ., UK; Peter Shternin (pshternin (at) gmail [dot] com) and Dima Yakovlev (yak (at) astro [dot] ioffe [dot] rssi [dot] ru) of Ioffe Technical Institute, Russia; and Dan Patnaude (patnaude (at) head [dot] cfa [dot] harvard [dot] edu) at the Smithsonian Astrophysical Observatory, Cambridge, MA, USA. We are issuing a press release in collaboration with my colleagues Dany Page (page (at) astro [dot] unam [dot] mx) at UNAM, Mexico City, Mexico; Madappa Prakash (prakash (at) harsha [dot] phy [dot] ohiou [dot] edu) at Ohio University, Athens OH, USA; James Lattimer (lattimer (at) mail [dot] astro [dot] sunysb [dot] edu) at SUNY Stony Brook, NY, USA; and Andrew Steiner (steinera (at) pa [dot] msu [dot] edu) at Michigan State Univ., East Lansing, MI, USA.

Links to the press releases and stories will be added below as they arrive.

Press releases (Feb. 23, 2011): Chandra press release, Chandra images
U. of Alberta release
Royal Astronomical Society (UK)
UNAM Institute de Astronomy (Mexico City), en espanol.

Stories: popular
Edmonton Journal (this version also in Vancouver Sun, Calgary Herald, etc.)
USA Today
Columbus (Ohio) Dispatch
Sify (India)
Popular Science (the video isn't worth it)
TG Daily
Milwaukee Journal Sentinel
Space Ref
Knight Science Journalism Tracker

Stories: more detailed Phys. Rev. Letters viewpoint article.
New Scientist
Science magazine
Science News

From our original identication of the cooling, in 2010. I've always wanted to have a news article about me start out with "It all started so well--fast cars, faster women, smashing up hotel rooms..."
And a blog that put our press release up verbatim: Claudio Marchesin's astronomy blog

Last modified: Mon Feb 28 14:43:06 MST 2011