Sat. Sep 24th, 2022
Image of a blue and white sphere, representing the star.
enlarge / An event on the surface of a magnetar can produce rapid radio bursts.

Dark matter turns out to be elusive. Other than the gravity evidence, which is strong, all other possible clues to it failed the examination. One problem could be that we just don’t know how to look for it, so detectors are based on informed guesses about how we might find dark matter. One approach to these searches is to look for places in the Universe that can generate a dark matter signal.

This is exactly the approach taken by some physicists in a recent study Physical Assessment Letter. In their case, they suggest that dark matter can produce a weak, but rather narrow radio signal with a bandwidth of 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 considered a possible signature of dark matter. But, as with all of these proposals, the work focuses on a particular version of dark matter.

Light axions seeking relief

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 well below one electron volt (by comparison, many dark matter proposals consist of particles with masses in the range of billions of electron volts). However, the axion has the advantage of being able to interact with photons under the right conditions. The problem (as is the case with all dark matter interactions) is that the signal from that interaction is very weak and hard to find.

However, neutron stars can provide some sort of amplification, making axions visible. A neutron star is a very compact object with a strong magnetic field. The magnetic field tears charged particles from the polar surface of the neutron star and accelerates them into space, generating very strong beams of radiation – this is the pulsar signal we associate with neutron stars.

Outside the poles, the magnetic field is more similar in shape to Earth’s magnetic field (but much, much stronger). In this region, the charged particles are trapped near the star, creating a plasma. The plasma becomes less dense the further away from the neutron star you go.

Light slows down in the presence of matter, including plasmas. So imagine a radio wave traveling out from somewhere above the surface of a neutron star. At first, it travels through a fairly dense plasma and moves quite slowly. However, the further away it gets, the faster it travels; eventually it reaches the speed of light in vacuum.

Likewise, an inward-traveling radio wave will slow down as it moves in the plasma, until it reaches the point where the plasma is so dense that it acts like a mirror. At that point, the radio wave is reflected and the delay is reversed.

Neutron star matchmakers

As we mentioned above, axions interact with light. In fact, axions can decay in a way that produces photons. The way this happens depends on the mass and energy distribution of the axion and its relationship to the speed and energy of the photon. When they match, the conversion is improved.

So think about it. The process of making a photon is like setting a swing in motion. Our axions have to give the push. Normally, axions hit randomly, so the swing is always moving, but not really swinging. If the axions hit the swing with the proper deceleration, the swing will begin to swing at its natural frequency and the amplitude will increase. This is an analogy: the axions have no 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 article are correct, it looks pretty amazing. Axions will float in space with a relatively narrow distribution of velocities. 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 pass a neutron star accelerate into or away from the star, passing through the plasma and 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 distance from the star, where the speed and mass of the axion match the speed and energy of radio waves. This process takes place for axions traveling both towards and away from the star. The axions traveling to the star generate a radio wave that is eventually bounced off the plasma, adding to the overall signal power.

In fact, the radio waves will be emitted within a fairly narrow energy band, meaning they should be easier to detect. And for neutron stars located near the center of a galaxy, the relatively high density of dark matter makes the process of generating radio waves a few orders of magnitude stronger.

Piggyback Study

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

On the other hand, this is a technique that is quite limited in terms of the stars it would work on. Pulsars emitting radio signals will most likely engulf any dark matter signal. So we need neutron stars that either don’t emit radio pulses in our direction, or emit pulses at higher frequencies than the radio waves we expect. They must also be close. Light traveling toward us from distant objects is Doppler shifted to longer wavelengths. If we go for very distant objects, the radio waves (broadcast at about 1 GHz) will be at very low frequencies by the time they get here, making them very difficult to pick out from the background.

On the plus side, this is the kind of observation that can piggyback on radio telescope sky surveys and slowly build data from multiple sources and multiple observations. Given that we’re going to have observatories like the square kilometer array producing more data than anyone knows how to handle, I think this is an excellent idea to follow up on.

Physical Assessment Letters2018, DOI: 10.1103/PhysRevLett.121.241102 (About DOIs)

By akfire1

Leave a Reply

Your email address will not be published.