A Nearby Supernova Could End The Search For Dark Matter

Eddie Gonzales Jr. – MessageToEagle.com – For 90 years, astronomers have struggled to understand dark matter, which makes up 85% of the universe’s matter but is invisible to telescopes. The leading candidate for dark matter is the axion, a lightweight particle that researchers worldwide are eager to find.

An artist’s concept of a highly magnetized neutron star. According to current theory, axions would be created in the hot interior of the neutron star. UC Berkeley astrophysicists say that the strong magnetic field of the star will transform these axions into gamma rays that can be detected from Earth, pinpointing the mass of the axion. Casey Reed, courtesy of Penn State

Astrophysicists at the University of California, Berkeley, suggest that axions could be discovered seconds after detecting gamma rays from a nearby supernova. If they exist, axions would be produced in large quantities during the first 10 seconds after a massive star collapses into a neutron star and would transform into high-energy gamma rays in the star’s strong magnetic field.

Such a detection is possible today only if the lone gamma-ray telescope in orbit, the Fermi Gamma-ray Space Telescope, is pointing in the direction of the supernova at the time it explodes. Given the telescope’s field of view, that is about one chance in 10.

Yet, a single detection of gamma rays would pinpoint the mass of the axion, in particular the so-called QCD axion, over a huge range of theoretical masses, including mass ranges now being scoured in experiments on Earth. The lack of a detection, however, would eliminate a large range of potential masses for the axion, and make most current dark matter searches irrelevant.

The problem is that, for the gamma rays to be bright enough to detect, the supernova has to be nearby—within our Milky Way galaxy or one of its satellite galaxies—and nearby stars explode only on average every few decades. The last nearby supernova was in 1987 in the Large Magellanic Cloud, one of the Milky Way’s satellites. At the time, a now defunct gamma-ray telescope, the Solar Maximum Mission, was pointing in the supernova’s direction, but it wasn’t sensitive enough to be able to detect the predicted intensity of gamma rays, according to the UC Berkeley team’s analysis.

“If we were to see a supernova, like supernova 1987A, with a modern gamma-ray telescope, we would be able to detect or rule out this QCD axion, this most interesting axion, across much of its parameter space—essentially the entire parameter space that cannot be probed in the laboratory, and much of the parameter space that can be probed in the laboratory, too,” said Benjamin Safdi, a UC Berkeley associate professor of physics and senior author of a paper that was published online Nov. 19 in the journal Physical Review Letters. “And it would all happen within 10 seconds.”

The researchers are anxious, however, that when the long-overdue supernova pops off in the nearby universe, we won’t be ready to see the gamma rays produced by axions. The scientists are now talking with colleagues who build gamma-ray telescopes to judge the feasibility of launching one or a fleet of such telescopes to cover 100% of the sky 24/7 and be assured of catching any gamma-ray burst. They have even proposed a name for their full-sky gamma-ray satellite constellation—the GALactic AXion Instrument for Supernova, or GALAXIS.

“I think all of us on this paper are stressed about there being a next supernova before we have the right instrumentation,” Safdi said. “It would be a real shame if a supernova went off tomorrow and we missed an opportunity to detect the axion—it might not come back for another 50 years.”

Initially, searches for dark matter targeted faint, massive compact halo objects (MACHOs) in the galaxy and cosmos. When these didn’t materialize, physicists turned to elementary particles theoretically around us, detectable in labs. These weakly interacting massive particles (WIMPs) also failed to appear.

The axion is the leading candidate for dark matter, fitting well within the standard physics model and addressing other particle physics puzzles. Axions also align with string theory, potentially unifying cosmic-scale gravity with quantum mechanics.

“It seems almost impossible to have a consistent theory of gravity combined with quantum mechanics that does not have particles like the axion,” Safdi said.

The leading candidate for an axion, the QCD axion, theoretically interacts weakly with all matter through the four forces of nature: gravity, electromagnetism, the strong force (holding atoms together), and the weak force (explaining atom breakup).

In a strong magnetic field, an axion can occasionally turn into a photon. Unlike neutrinos, which interact only through gravity and the weak force, axions can interact with the electromagnetic force.

Lab bench experiments like the ALPHA Consortium, DMradio, and ABRACADABRA—with UC Berkeley researchers—use compact cavities that resonate and amplify the faint electromagnetic field or photon produced when a low-mass axion transforms in a strong magnetic field.

Astrophysicists suggest searching for axions from neutron stars post-core-collapse supernova, like 1987A. Previously, they focused on detecting gamma rays from axions’ slow transformation into photons in galaxies’ magnetic fields. Safdi and colleagues realized this process is inefficient for producing detectable gamma rays on Earth.

They explored gamma ray production by axions in strong magnetic fields around the star that generated them. Supercomputer simulations showed this creates a gamma ray burst dependent on the axion mass, occurring simultaneously with a neutrino burst from inside the neutron star. This axion burst lasts only 10 seconds after the neutron star forms, then the production rate drops dramatically, hours before the outer layers explode.

“This has really led us to thinking about neutron stars as optimal targets for searching for axions as axion laboratories,” Safdi said. “Neutron stars have a lot of things going for them. They are extremely hot objects. They also host very strong magnetic fields. The strongest magnetic fields in our universe are found around neutron stars, such as magnetars, which have magnetic fields tens of billions of times stronger than anything we can build in the laboratory. That helps convert these axions into observable signals.”

Two years ago, Safdi and his colleagues put the best upper limit on the mass of the QCD axion at about 16 million electron volts, or about 32 times less than the mass of the electron. This was based on the cooling rate of neutron stars, which would cool faster if axions were produced along with neutrinos inside these hot, compact bodies.

The study describes the production of gamma rays following core collapse to a neutron star, but also uses the non-detection of gamma rays from the 1987A supernova to put the best constraints yet on the mass of axion-like particles, which differ from QCD axions in that they do not interact via the strong force.

They predict that a gamma ray detection would allow them to identify the QCD axion mass if it is above 50 microelectron volts (micro-eV, or µeV), or about one 10-billionth the mass of the electron. A single detection could refocus existing experiments to confirm the mass of the axion, Safdi said. While a fleet of dedicated gamma-ray telescopes is the best option for detecting gamma rays from a nearby supernova, a lucky break with Fermi would be even better.

“The best-case scenario for axions is Fermi catches a supernova. It’s just that the chance of that is small,” Safdi said. “But if Fermi saw it, we’d be able to measure its mass. We’d be able to measure its interaction strength. We’d be able to determine everything we need to know about the axion, and we’d be incredibly confident in the signal because there’s no ordinary matter which could create such an event.”

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Written by Eddie Gonzales  Jr. – MessageToEagle.com Staff Writer