Astrophysicists suggest that a supernova within the Milky Way or a satellite galaxy could produce a flood of axions, a leading dark matter candidate particle. If axions exist, they would emerge in massive quantities during the first ten seconds of a massive star's collapse into a neutron star. As they travel outward, the axions would encounter the star's magnetic field, transforming into high-energy gamma rays detectable from Earth.
"Detection of a single gamma-ray burst could identify the mass of the QCD axion across a wide theoretical range," said Benjamin Safdi, associate professor of physics at UC Berkeley. "It would also rule out much of the mass range current experiments are investigating."
However, capturing this event is challenging. The supernova must occur nearby, and the gamma-ray telescope must be correctly positioned. With only one operational gamma-ray observatory, the Fermi Gamma-ray Space Telescope, the chance of capturing such an event is roughly one in ten. Adding to the difficulty, nearby supernovae are rare, occurring approximately once every few decades. The last such event was the 1987A supernova in the Large Magellanic Cloud, which occurred before modern gamma-ray detectors were sensitive enough for axion-related observations.
The UC Berkeley team is advocating for the development of a full-sky gamma-ray satellite array called GALAXIS (GALactic AXion Instrument for Supernova). This would ensure continuous monitoring of the sky, increasing the likelihood of detecting gamma rays from future supernovae.
"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."
Axion Theory and Detection Challenges
Axions are highly promising candidates for dark matter. These particles align with the standard model of physics and address unsolved mysteries in particle physics. Unlike neutrinos, which interact weakly with gravity and the weak force, axions are expected to interact faintly with all four fundamental forces, including electromagnetism.
Laboratory searches for axions focus on their potential to transform into photons in strong magnetic fields. Experiments like ALPHA, DMradio, and ABRACADABRA aim to detect these transformations in controlled environments. However, Safdi and colleagues argue that neutron stars offer an unparalleled natural laboratory. Their intense magnetic fields and extreme temperatures create conditions ideal for producing observable axion-related gamma rays.
"This has really led us to thinking about neutron stars as optimal targets for searching for axions as axion laboratories," Safdi explained.
Potential for Gamma-Ray Breakthroughs
Using simulations, the team found that axions from a supernova would produce a gamma-ray burst lasting only 10 seconds. Such an observation would yield vital information, including the mass of the QCD axion, potentially narrowing down the parameter space currently being explored in laboratories.
The researchers also revisited data from the 1987A supernova, which lacked the sensitivity to detect axion-related gamma rays. Their analysis helped refine constraints on axion-like particles, providing valuable insights for future experiments.
"The best-case scenario for axions is Fermi catches a supernova," Safdi noted. "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."
The UC Berkeley team includes graduate student Yujin Park and postdoctoral fellows Claudio Andrea Manzari and Inbar Savoray. Their study was published in 'Physical Review Letters'.
Research Report:Supernova Axions Convert to Gamma Rays in Magnetic Fields of Progenitor Stars
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