Axions are considered the most promising candidates for the mysterious dark matter that fills our universe. Astrophysicists are actively looking for signals of high-mass axions that may be generated during supernova explosions. Researchers suggest that a fast track to detect these axions is by observing a gamma-ray burst that coincides with a neutrino burst from a nearby core-collapse supernova. However, to successfully capture these infrequent occurrences, we would require a network of gamma-ray telescopes.
The quest to uncover the dark matter of the universe could culminate tomorrow, provided a supernova occurs nearby and luck is on our side.
The true nature of dark matter has puzzled astronomers for nearly a century, based on the fact that 85% of the matter in the universe eludes detection by our telescopes. Currently, the most favored candidate for dark matter is the axion, a lightweight particle that scientists worldwide are eager to discover.
Astrophysicists at the University of California, Berkeley assert that we could detect axions just seconds after gamma rays from a nearby supernova explosion are observed. If axions indeed exist, they would be produced in large numbers within the first 10 seconds following the core collapse of a massive star into a neutron star, escaping and transforming into high-energy gamma rays thanks to the star’s powerful magnetic field.
Currently, such a detection is contingent on the single gamma-ray telescope in orbit—the Fermi Gamma-ray Space Telescope—being aimed at the supernova explosion when it occurs. Given the telescope’s viewing range, this presents roughly a 10% chance.
Nevertheless, a successful detection of gamma rays would establish the mass of the axion, particularly the QCD axion, across a vast spectrum of theoretical masses, some of which are being tested in Earth-based experiments. Conversely, a lack of detection would eliminate a significant range of potential axion masses, making many ongoing searches for dark matter irrelevant.
The challenge lies in the fact that for the gamma rays to be detectable, the supernova must be relatively close—within our Milky Way galaxy or its satellite galaxies. Nearby supernova occurrences happen only once every few decades on average. The last one, Supernova 1987A, occurred in 1987 in the Large Magellanic Cloud, one of the Milky Way’s companions. At that time, a now-retired gamma-ray telescope, the Solar Maximum Mission, was directed toward the explosion, but was not sensitive enough to detect the predicted gamma-ray intensity, according to the analysis from the UC Berkeley team.
“If we were to observe a supernova like 1987A with a modern gamma-ray telescope, we could either detect or rule out this fascinating QCD axion across a vast part of its parameter space—encompassing nearly all regions that are untestable in laboratory settings, as well as much of the regions that are,” stated Benjamin Safdi, a UC Berkeley associate professor of physics and the lead author of a paper released online on November 19 in the journal Physical Review Letters. “And it would all transpire within just 10 seconds.”
However, the researchers are concerned that if a long-anticipated supernova occurs in the nearby universe, they may not be adequately prepared to observe the gamma rays generated by the axions. The team is presently collaborating with colleagues who design gamma-ray telescopes to evaluate the possibility of launching a single telescope or a group of telescopes capable of monitoring 100% of the sky around the clock, ensuring any gamma-ray burst is detected. They have even proposed a name for their proposed gamma-ray satellite network—GALactic AXion Instrument for Supernova, or GALAXIS.
“We are all somewhat anxious that a supernova may occur before we have sufficient instruments ready,” Safdi remarked. “It would be quite unfortunate if a supernova exploded tomorrow and we missed the opportunity to detect the axion—it may be another 50 years before such an event occurs again.”
Joining Safdi are co-authors graduate student Yujin Park and postdoctoral researchers Claudio Andrea Manzari and Inbar Savoray, all affiliated with the UC Berkeley physics department and the Theoretical Physics Group at Lawrence Berkeley National Laboratory.
QCD axions
Initial dark matter searches centered on faint, massive compact halo objects (MACHOs) spread throughout our galaxy and beyond. However, after these were not found, scientists began investigating elementary particles that should be detectable in Earth-based laboratories. Unfortunately, these weakly interacting massive particles (WIMPs) also failed to present evidence. Currently, axions stand as the leading candidate for dark matter, fitting well into the standard model of physics while addressing several unresolved issues in particle physics. Axions also arise from string theory, a proposition concerning the universe’s underlying structure, and may help unify gravity, which governs cosmic interactions, with quantum mechanics, which deals with incredibly small scales.
“It seems nearly impossible to develop a consistent theory combining gravity with quantum mechanics without including particles like the axion,” Safdi explained.
The most favored axion type, the QCD axion—named after the prevailing theory of the strong force, quantum chromodynamics— is believed to interact weakly with all matter through the four forces of nature: gravity, electromagnetism, the strong force, which binds atoms together, and the weak force, associated with atomic decay. One outcome of these interactions is that in strong magnetic fields, axions may occasionally convert into electromagnetic waves or photons. Axions differ significantly from another lightweight and weakly interacting particle, the neutrino, which interacts solely through gravity and the weak force, ignoring the electromagnetic force entirely.
Experiments conducted in laboratories—such as the ALPHA Consortium (Axion Longitudinal Plasma HAloscope), DMradio, and ABRACADABRA, all involving UC Berkeley researchers—utilize compact cavities that resonate and amplify the faint electromagnetic field or photon generated when a low-mass axion shifts in the presence of a strong magnetic field.
Alternatively, astrophysicists have proposed searching for axions formed inside neutron stars shortly after a core-collapse supernova like 1987A. Until now, their focus has primarily been on detecting gamma rays from the gradual transformation of these axions into photons within the magnetic fields of galaxies. However, Safdi and his collaborators concluded that this method is not particularly effective for producing detectable gamma rays.
Instead, they investigated the generation of gamma rays by axions in the strong magnetic fields surrounding the very star that produced them. Supercomputer simulations showed that this process efficiently creates a burst of gamma rays dependent on the axion’s mass, coinciding with a neutrino burst from inside the hot neutron star. Unfortunately, this axion burst only lasts a mere 10 seconds after the neutron star forms, after which production rates plummet, even though the star’s outer layers may explode hours later.
“This has led us to consider neutron stars as ideal targets for searching for axions, effectively ‘axion laboratories’” Safdi noted. “Neutron stars are remarkably hot and possess extraordinarily strong magnetic fields—the strongest known in the universe. Magnetars, for example, have magnetic fields tens of billions of times more powerful than what we can create in laboratories, facilitating the conversion of axions into detectable signals.”
Two years ago, Safdi and his colleagues placed the best upper limit on the mass of the QCD axion at approximately 16 million electron volts or about 32 times lighter than an electron. This assessment was influenced by the cooling rate of neutron stars, which would cool more quickly if axions were produced alongside neutrinos within these hot, dense bodies.
In their current research, the UC Berkeley team not only discusses the formation of gamma rays following the core collapse of a neutron star but also uses the absence of gamma rays from the supernova 1987A to place the strongest constraints yet on the mass of axion-like particles that differ from QCD axions in not interacting via the strong force.
They predict that detecting gamma rays could help determine the QCD axion’s mass if it exceeds 50 microelectrons volts (micro-eV, or μeV), which is about one ten-billionth the mass of an electron. A successful observation could realign existing experiments aimed at confirming the axion’s mass, according to Safdi. While creating a fleet of dedicated gamma-ray telescopes remains the optimal route for sensing gamma rays from a nearby supernova, receiving a fortunate signal from Fermi could be even more advantageous.
“The ideal outcome for axion detection is that Fermi captures a supernova event. The probabilities are slim, though,” Safdi stated. “However, if Fermi detected it, we could measure its mass, strength of interaction, and gain a comprehensive understanding of the axion, ensuring strong confidence in the signal as no ordinary matter could produce such an event.”
This research received funding from the U.S. Department of Energy.
