The Neutron Star Cooling Mystery

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How can we probe the interiors of neutron stars? Astronomers Wynn Ho and Craig Heinke have approached this tough question using the Chandra X-Ray Observatory. They monitored a nearby neutron star at the center of the Cassiopeia supernova remnant (created by a supernova that occurred in the 1680s), a mere 11,000 light-years away. However, their work has created more questions than answers.

Neutron stars are some of the most exotic objects in our universe. They are incredibly dense, such that if you could scoop out a bit of neutron star material with a thimble, it would weigh as much as a mountain. They are so compact that a neutron star with the same mass as our sun would measure the size of a city!

With densities such as these, the interiors of these dead stars are truly bizarre. As the name suggests, they are made almost entirely of neutral particles that are normally found inside the nuclei of atoms. But no proper atoms can survive inside these stars because they’re packed in so very tightly.

We don’t fully understand the structure of neutron stars’ interiors, but there are some interesting theories. For example, the neutrons may act as a superfluid, which is an unusual situation where a fluid flows without friction or viscosity. If you rotate a cup with a superfluid inside, the fluid itself will stay still, apparently ignoring the force of gravity.

Ho presented temperature measurements over a 10-year period with Chandra at the recent Royal Astronomical Society’s National Astronomy Meeting. The rate at which the neutron star cools tells us something about the interior, since young neutron stars cool by radiating neutrinos, very light particles that hardly interact with matter, from deep inside the star. These observations help rule out certain models of neutron star cooling and interiors.

However, the scientists noticed the neutron star cooled in 2006, a change in temperature they didn’t expect. Since the original press release was written, the researchers reprocessed the data and realized the observed cooling was real. Heinke says that they can’t yet explain why this cooling happened. Some other process may be affecting the neutron star, so they will be investigating further.

In the end, we have more questions than answers after careful observation, as often happens in astronomy.

Thanks to Craig Heinke for more info on this story and answering my questions!

Image Credit: NASA/CXC/Southampton/W. Ho et al.; Illustration: NASA/CXC/M.Weiss