Dark matter is proving elusive. Apart from the gravitational evidence, which is strong, all the other potential indications of it haven't held up to scrutiny. One issue may be that we simply don’t know how to look for it, so detectors are based on informed guesses about how we might expect to find dark matter. One approach to these searches is to look for places in the Universe that might generate a dark matter signal.

This is exactly the approach taken by some physicists in a recent Physical Review Letter. In their case, they suggest that dark matter might produce a weak, but quite narrow, bandwidth radio signal from neutron stars.

This team is not the first to propose looking for dark matter signatures in the Universe. Excess gamma rays from the center of our own galaxy were, for a while, thought to be a possible signature of dark matter. But, as with all these proposals, the work focuses on a particular version of dark matter.

Light axions looking for enlightenment

The version of dark matter that is the focus of this work is a hypothetical subatomic particle known as the axion. The axion is very light, with proposed masses going way below one electron volt (in comparison, many dark matter proposals consist of particles with masses in the billion electron volt range). The axion, though, has the advantage that it can, under the right circumstances, interact with photons. The problem (as is the case with all dark matter interactions) is that the signal of that interaction is really weak and hard to find.

However, neutron stars may provide a sort of amplification, making axions visible. A neutron star is a very dense object that has a strong magnetic field. The magnetic field rips charged particles off the neutron star polar surface and accelerates them out into space, generating very strong beams of radiation—this is the pulsar signal that we associate with neutron stars.

Away from the poles, the magnetic field looks more like the Earth’s magnetic field in terms of its shape (but much, much stronger). In this region, the charged particles are trapped in the vicinity of the star, making a plasma. The plasma gets less dense the farther away from the neutron star you go.

Light slows down in the presence of matter, including plasmas. So, picture a radio wave traveling outward from somewhere above the surface of a neutron star. At first, it is traveling through a reasonably dense plasma and moves rather slowly. The farther away it gets, though, the speedier it travels; eventually, it reaches the speed of light in vacuum.

Likewise, a radio wave traveling inward will slow as it moves into the plasma, until it hits the point where the plasma is so dense that it acts like a mirror. At that point, the radio wave will be reflected, and the deceleration will be reversed.

Neutron star matchmakers

As we mentioned above, axions interact with light. In fact, axions can decay in a way that produces photons. Critically, the way this happens depends on the axion’s mass and energy distribution and its relationship to the photon's speed and energy. When these match, conversion is enhanced.

Think of it like this. The process of making a photon is like setting a swing in motion. Our axions have to provide the push. Normally, axions just hit at random, so the swing is always moving but not actually swinging. If the axions hit the swing with the right time delay, then the swing will start to swing at its natural frequency, and the amplitude will grow. This is an analogy: the axions don't have a specific timing with each other. But, the process works because of the relationship between the light wave and the axion.

If the calculations in the new paper are correct, it looks pretty amazing. Axions will be floating around in space with a relatively narrow distribution of speeds. This is because dark matter is trapped in galaxies, which wouldn’t happen if it moved too fast, meaning dark matter must be cold and move slowly.

Axions that happen to be passing a neutron star will accelerate into or away from the star, passing through the plasma and the strong magnetic field along the way. The strong magnetic field allows the axion to decay into a photon. However, this decay will be most efficient at a certain range of distances from the star, where the speed and mass of the axion will match the speed and energy of radio waves. This process happens for axions traveling both toward and away from the star. The axions traveling toward the star generate a radio wave that is eventually reflected by the plasma, adding to the total signal power.

Even better, the radio waves will be emitted within a rather narrow energy band, meaning that they should be easier to detect. And, for neutron stars sitting near the center of a galaxy, the relatively high density of dark matter makes the radio-wave-generation process a few orders of magnitude stronger.

Piggyback research

The researchers performed some basic calculations for nearby neutron stars, and they came to the conclusion that at least two of them should provide signal strengths that fall within the sensitivity limit of current radio telescopes.

On the downside, this is a technique that is pretty limited in terms of the stars it would work on. Pulsars that emit radio signals will, most likely, swamp any dark matter signal. So, we need neutron stars that either do not emit radio pulses in our direction or emit pulses at higher frequencies than the radio waves we expect. They also have to be close. Light traveling to us from distant objects is Doppler shifted to longer wavelengths. If we are to go for very distant objects, the radio waves (emitted at around 1GHz) will be at very low frequencies by the time they get here, making them very difficult to pull out of the background.

On the upside, this is the sort of observation that can piggyback on radio telescope sky surveys and slowly build up data from multiple sources and multiple observations. Considering we are going to have observatories like the square kilometer array producing more data than anyone knows how to cope with, I think this is an excellent idea to follow-up on.

Physical Review Letters, 2018, DOI: 10.1103/PhysRevLett.121.241102 (About DOIs)