A type of star that I hold dear is the pulsar (a type of neutron star). Not just because the first one discovered was called LGM-1 (Little Green Men), but because they are a rich mixture of quantum physics, electromagnetism, and gravity, all in a single macroscopic object. For a young graduate student, being able to solve a set of equations and describe (at a simple level) the behavior of an entire freaking star is just mind-blowing.

But they are also a huge mystery, having a complex structure and possibly mountains. Another mystery that's not inherent to the objects themselves is that neutron stars of a certain type are conspicuous by their absence in the galactic center. There are many possible reasons for this absence—maybe there aren't any good schools in the neighborhood or it's too far to the local pub—but one of the most exciting possibilities is that the heaviest neutron stars are being hunted down and devoured by dark matter.

Neutron stars are essentially the corpses of stars. After burning through all their fuel and exploding in a last furious burst of energy, the remaining matter collapses in on itself. The temperature and pressure get so high that the electrons and protons fuse to form neutrons. However, their mass isn't sufficient for gravity to force the neutrons together—if it were, a black hole would form.



The pressure that prevents a neutron star from collapsing is called the Fermi pressure. Neutrons are fermions, which means they repel each other. Fermions cannot occupy the same quantum state, so at close range, they stack in energy and space themselves out. This unusual state also generates huge magnetic fields, which accelerates charged particles to enormous energies as the star spins. These particles emit beams of radiation that sweep around like the beam from a light house. When we happen to fall in the path of this beam, we record this as a regular blip of light.

To put this rotation in perspective: a neutron star is considerably smaller than Earth, has more mass than the Sun, and can be rotating more than 100 times per second.



The age and mass distribution of neutron stars represents a sort of fossil record for stars, both in our galaxy and in the Universe generally. But, then, just like the fossil record, there are gaps. In the case of neutron stars, the most obvious gap is that there are almost no old pulsars with fast rotations near the galactic center. These represent the very heaviest of pulsars, and it seems likely that they should be well represented near the galactic center.

There is a potential explanation for their apparent absence. Researchers have proposed that because the radiation had to travel through clouds of charged particles—mainly electrons—before it gets to us, the radio pulses would be stretched out. The stretching may be so great that we simply cannot observe them. The pulsars are there but invisible.



Recently, though, a pulsar near the galactic center was found. Observations have shown that the pulses are not stretched out very much at all. Now, this pulsar is a rather young and vigorous fellow. Its easily visible presence, however, re-opens the question: where are all the old fast-spinning pulsars?

One possibility is that the missing pulsars are simply not there. The missing pulsars are heavy, so some extra mass could turn into black holes. Ordinary matter accumulates on the outside of the star and cannot cause this to happen. But dark matter might accumulate at the center of the neutron star, where it could cause the neutron star to collapse into a black hole.

The cool thing about this idea is that it is sort of self constraining. Let's imagine that dark matter did collect in the center of neutron stars, but very, very slowly. Then, yes, a black hole might form, but it would be tiny. Tiny black holes radiate lots of Hawking radiation, so the new black hole can't suck in matter faster than it loses energy. As a result, it would evaporate in a burst of Hawking radiation. Hence, if black holes do form, then we can calculate a minimum strength for the interaction of dark matter.

On the other hand, if dark matter accumulates too fast, then, even at locations away from the galactic center, old fast-spinning neutron stars should not exist anywhere. Hence, there is a maximum interaction strength as well.

In addition, the age of fast-spinning neutron stars should be correlated to the dark matter density. And, finally, all the constraints above should not be completely in contradiction to constraints on dark matter's properties derived from other measurements, such as those from the bullet cluster.

To calculate these constraints, the researchers used the age of the newly found pulsar near the galactic center and a very old fast-spinning pulsar that is at a similar distance from galactic center as the Earth. They find a surprisingly narrow range of interaction strengths as a function of the mass of dark matter particles. The constraints don't completely contradict pre-existing data, which is nice.

What is really needed, though, is statistics using many pulsars and neutron stars at different distances and in different dark matter densities. This hasn't been done in this paper. It may simply be impossible, because there may not be enough pulsars of the correct type to obtain reasonable statistics.

Assuming that there are enough pulsars to determine if there is a correlation between expected dark matter concentrations and the absence of high mass pulsars, finding a correlation would be good evidence that dark matter does have an appetite for heavy neutron stars and provide us with better information about the properties of dark matter. If that were true then the search space for dark matter particles would be considerably narrowed, making life easier for the folk at the LHC.

Physical Review Letters, 2014, DOI: 10.1103/PhysRevLett.113.191301