A collision of two extraordinarily dense, collapsed stars in the distant universe is providing potential clues to the axion, a dark matter candidate first proposed half a century ago.
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The stellar remnants are neutron stars, the corpses that remain after massive stars collapse in on themselves. These dead stars are so dense that their electrons collapse onto their protons—hence, “neutron star.” Their extreme density also makes them a venue for exotic physics: specifically, they’ve been proposed as a source of axions, a hypothetical particle that could contribute to the universe’s dark matter content.
New research, published earlier this month in Physical Review Letters, puts constraints on how axion-like particles might couple with photons, based on spectral and temporal data from a neutron star merger roughly 130 million light-years away.
Axion-like particles (or ALPs) are a more general class of hypothetical dark matter candidates than axions, and scientists believe their nature could be revealed by studying photons and constraining the mass range of the particles. The axion-like particles produced in the neutron star merger escape the remnant and decay back into two photons, the team wrote in the paper, producing an electromagnetic signal detectable to telescopes. The data was collected from 2017 observations of the collision taken by the Fermi Large Area Telescope (Fermi-LAT).
“For a neutron star merger, there’s a unique opportunity where you could get the photon signal,” said Bhupal Dev, a physicist at Washington University in St. Louis and lead author of the study, in a phone call with Gizmodo. “We could utilize this multimessenger study, this data, to probe some new physics beyond the Standard Model.”
Dark matter appears to constitute 27% of the universe, but it interacts so weakly with ordinary matter that scientists can only detect it through its gravitational effects on what we can see. Popular dark matter candidates (which is to say, theorized responsible parties for dark matter’s apparent existence) are Weakly Interacting Massive Particles (WIMPs), hidden (or dark) photons, massive compact halo objects (MACHOs), and, of course, axions.
Named for a brand of laundry detergent, the axion is a hypothetical particle that was proposed in the 1970s as a solution to physics’ strong-CP problem, which describes the fact that quarks’ adherence to the laws of physics remains the same, even when the particles are replaced with their mirror images.
Neutron stars are some of the densest objects in the universe, beaten only by black holes. Unlike black holes, light can escape neutron stars, making them observable on the electromagnetic spectrum.
Dev explains that axions could arise from neutron star mergers in a couple of ways, if axions indeed couple to photons. Through photon coalescence, axions would emerge from photons coming together in the intensely hot astrophysical environment and fusing. The other way axions could arise is through the Primakoff process, in which a photon interacts with a bath of electrons, producing axions.
The axion, as it’s proposed, is so small that it would sometimes behave more like a wave than a particle, meaning it flees the scene of the crime with relative ease. But the proton is (relatively) massive, so it takes a moment for the particle to emerge from this hotbed of interaction. Specifically, it takes 1.7 seconds: the amount of delay the researchers observed between the gravitational wave signal from a neutron star merger and the electromagnetic signal from it.
“We get a lot of photons from the sky. So how do we really know that this photon signal is coming from the axion?” Dev said. “This is coming from a decay of the particle, versus astrophysical processes where the photons disappear from scattering. So there is a difference in the spectrum. We can analyze both the timing information and we can also analyze the spectral features. And that’s where we can disentangle these kinds of new physics signals from the standard astrophysical processes.”
Earth-based experiments are also working to narrow the potential mass ranges of the axion. LUX-Zeplin, XENON-1T, and the ALPS II experiment, which began operations in May 2023, are all designed to seek out axions deep underground. But there are also other projects, like ADMX and the Dark Matter Radio Pathfinder, working to constrain the mass range on hidden (or dark) photons, another class of dark matter candidates. Later generations of the Dark Matter Radio will hunt axions.
The new research “gives some new constraints on the axion-like particles, because so far we did not see any signal of axions,” Dev said. “It also gives us hope that in the future, using these astrophysical observations, we could gain more insight into axion-like particles. And this will be complementary to the laboratory searches that are going on.”
The hunt for axions is a lot like using a metal detector on a very, very large beach. More often than not, physicists and astronomers are detecting nothing. But searching the full range of potential masses for axions and axion-like particles is the best way to eventually track them down.
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