Neutron stars are the most dense form of matter in our Universe (black holes cram more stuff into a smaller space, and it's not clear if that stuff is still "matter"). A neutron star is produced by the collapse of a stellar core, which crams a bit more mass than our Sun into a sphere about 20 kilometers across.
At this density, matter does strange things. Models based on theoretical considerations suggest that there's a distinct "crust" that sits atop a superfluid of subatomic particles, but it's not like we can visit one and confirm this. Now, researchers have done the next-best thing: they've arranged for a telescope to stare at a neutron star for three years, waiting for it to undergo a "glitch" in its normal behavior. The results give us one of our first direct tests of competing models for what's beneath the surface of a neutron star.
While a neutron star is composed primarily of neutrons (duh!), there are also protons present in its interior. All the particles there form a superfluid, which can flow without any friction. The flow of these charged particles inside the star can create an intense magnetic field, one that can accelerate charged particles near the star and cause them to emit photons. The rapid rotation of the star means that these jets of charged particles sweep a large area of space with the photons they produce. On Earth, we see this as a flash of light appearing from the same source many times a second—a pulsar. The pulses of photons that give these stars their name arrive with such regularity that we've used them as an extremely precise test of relativity.
But the regularity does have its limits. The same magnetic fields that power the pulsar produce a bit of drag as they sweep across the environment, gradually slowing the pulsar down. And theorists have proposed that neutron stars can "glitch," experiencing a sudden speed-up. This occurs due to movement in the star's interior, which can exchange momentum between the superfluid there and the crust surrounding it. Until now, however, our understanding of glitches had remained limited to theory.
To understand glitches, a team of astronomers arranged to track the Vela pulsar for a period of three years using two radio telescopes (the Mount Pleasant observatory in Tasmania and the Ceduna Observatory in Australia. During those three years, the astronomers observed a grand total of one glitch. In a first, they managed to catch both the glitch and every pulse that surrounded it, along with the polarization of the light in each pulse.
The event lasted just a fraction of a second and was presaged by a weak and very broad pulse. Ninety milliseconds later, when the next pulse was expected to arrive, nothing happened. The next few pulses were weak and had little indication of the strong polarization that was seen in the pulses that arrived before the glitch. Checking through 100,000 pulses that were recorded during their observations showed there was nothing like this behavior in the records.
A model competition
An analysis of the data surrounding the glitch revealed that the mean length of time between pulses had gradually increased for a few seconds prior to the glitch. The researchers suggest that this is the product of changes in the interior of the neutron star, as a superfluid vortex became "unpinned" from the crust above it. The change in timing was either the process of transferring momentum to the crust or the result of the vortex altering the magnetic flux lines of the neutron star.
Critically, this time (4.4 seconds) can be predicted by the equation of state that we use to describe the conditions inside a neutron star. And 4.4 seconds is apparently consistent with an equation called the density-dependent hadronic model, which means that this is our first chance to test some of the models that explain glitching against real-world data.
Unfortunately, as this work makes very clear, glitches are rare events, and it takes a lot of time to capture all the data relevant to them. As a result, it will likely take some time before we can have additional observations that provide further tests and tell us whether the behavior of Vela is typical of pulsars. But the chance to peel back the crust and look at the exotic conditions inside a neutron star is probably going to be tempting enough to ensure that it'll happen.