Are neutron stars superconductors?

In 2011, came a stunning astrophysical discovery, that changed the way how scientists look at the interior of neutron stars. Astronomers tracked the cooling of a pulsar in the constellation Casseopaeia over a period of ten years.

Superfluidity is a mind-boggling phenomena that was first discovered in liquid Helium. At very low temperatures, fluids that possess this property flow without any resistance. This led to large scale applications in industry, and led to an in-depth research in this field. Superfluids were found to display several bizarre properties and were explored widely in cryogenic laboratories. According to BCS Theory (after Bardeen–Cooper–Schrieffer who proposed it), the properties are attributed to the pairing of electrons ("Cooper pairs").

Surprisingly, it has been proposed that neutron stars may possess superfluidity in their interior. It is claimed that at the high density and low temperatures (compared to energy of excitation of particles) that exist in the interior of neutron stars, nuclear interactions may lead to formation of such Cooper pairs, leading to superfluidity.

The presence of superfluidity in the neutron star core cannot be probed directly, but scientists try to find astrophysical observations that may contain the hidden signature of such effects. It has been suggested that this phenomena may be responsible for observed periodic "glitches" in rotation frequencies of neutron stars. In general, neutron stars are extremely regular rotators, except when some of them show occasional jumps or glitches, which then slowly relaxes towards the normal value. It has been seen in laboratory experiments that rotation of superfluids lead to formation of vortices. It is thought that neutron stars may also have superfluid vortices in their interior. These vortices may creep outward gradually and snap, causing transfer in energy from crust to the core, explaining such glitches.

In 2010, cooling of the neutron star in the constellation Casseopeia A was first reported by Craig Heinke, from the University of Alberta, Canada, and Wynn Ho from the University of Southampton, UK. They analyzed NASA's Chandra X-ray observatory data and found a rapid fall in the temperature, showing that it had cooled by a few percent over a 10-year period. It was the first time that astronomers have measured the rate of cooling of a young neutron star.

Two independent groups, led by Dany Page of National Autonomous University in Mexico and Peter Shternin of Ioffe Institute in St Petersburg, Russia, suggested that this peculiar fast cooling can be explained by invoking Cooper pair formation in the neutron star core, leading to superfluidity.

Although other groups have suggested alternative explanations for the cooling observations, the invoking of superfluidity is one of the most promising. However, other groups have yet to confirm the cooling behaviour over a longer observation period. If confirmed, this would be one of the major discoveries of superfluidity in astrophysical systems.