One of the hardest parts of science is determining the difference between "not yet observed" and "doesn't actually exist." After all, just one more observation might turn up the unexpected, right? There are two big factors that go into making the decision. One is the accumulation of enough data that we can be relatively sure that the missing data doesn't really exist. The second is a plausible model that tells us that the data will never show up.

X-Ray pulsars may present a case in point. We have been observing X-Ray pulsars and their familial relations for the last 45 years. But all the X-Ray pulsars that we might expect to flash slowly seem to be missing. A recent paper in Nature Physics adds to that body of data with a model that explains why these long-period X-Ray pulsars are missing.

Pulsars are neutron stars—remnants from supernova explosions. Neutron stars have immensely huge magnetic fields (think ~1012 times Earth's magnetic field). These fields accelerate charged particles, and in the course of that acceleration, light is emitted. But because of the nature of the fields, the light is emitted in a rather narrow cone. Because neutron stars rotate, this cone is scanned like a search light across the sky. So we only observe pulsars if the powerful beam happens to sweep across the face of the Earth.

Most pulsars emit light in the microwave region, and they spin extremely fast, with periods ranging from a few seconds down to 100 ms. Some neutron stars, however, have magnetic fields that are an order of magnitude or two larger than ordinary. These stars emit X-Rays and gamma rays and are generally pretty fearsome. If we were to expect these pulsars to spin down gradually (slowing because of the processes that are emitting all that energy), then we should find them with rotational periods that cover a large range, with older ones rotating more slowly on average.

Instead, they cluster in a range between two and 12 seconds. This seems a bit bizarre, because given the age of the stars and their current rotational period, we should find stars marginally older (a few thousand years, which is the blink of an eye star-wise) that have periods of around half a minute. But they have yet to be found.

Why don't we find them? What it boils down to is that the electronic properties of the star's crust dominate the spin-down of the star. The crust of a neutron star is about 1 km thick and is quite crystalline near the outer layers. However, the deeper into the crust we go, the less well-ordered it is, until we reach a zone called the nuclear pasta region. It got that name because it is thought to consist of ordered regions that form sheets (lasagna), tubes (macaroni), and wires (spaghetti). Importantly, there is no direct evidence for this region; instead it's the outcome of modelling how the energy, pressure, and density of the star affect the ordering of its constituents.

This pasta has a distinctive property: it changes the way energy is dissipated and transported within the star. The magnetic field generates currents in the pasta region, which provides an intermediate step in converting magnetic energy to rotational energy. Hence, the crust changes the way the star spins down. In a series of models that take different neutron star masses, different crust diameters, and differently pasta region sizes, researchers from Spain showed that without a pasta region of some kind, a neutron star continues to spin down indefinitely, and we should observe X-Ray pulsars with periods that extend out past one minute.

The pasta, however, disrupts the magnetic field, stealing energy from it. In the end, that energy is transferred to rotational energy, keeping the spin period up. This is not such an efficient process, though, so for the early stages of the neutron star's life, it rapidly spins down. This continues until the additional energy from the magnetic field counters the losses due to other processes, stabilizing the rotational period at the cost of the magnetic field. The exact period at which this occurs depends on the mass of the star, the thickness of the crust, and the fraction of impurities in the crust.

What's important is that the rotation period still clusters between seven and 40 seconds. If the pasta region is removed, no such clustering occurs.

In some sense this isn't news at all, because astronomers knew that something must be stabilizing the rotational periods of X-Ray pulsars. But the nice thing about this model is that it shows indirect evidence for the pasta region in a neutron star and simultaneously provides a natural explanation for observed data. What the model does not do is tell us much about the pasta region. For instance, the researchers can get the same result by assuming that the entire inner crust area is simply disordered (presumably by impurities). However, as larger numbers of X-Ray pulsars are discovered, the statistics of their spin periods will allow these models to be distinguished.

Nature Physics, 2013, DOI: 10.1038/nphys2640