For over a decade, astronomers have puzzled over the origins of fast radio bursts, brief blasts of radio waves that come mostly from distant galaxies. During that same period, scientists have also detected high-energy neutrinos, ghostly particles from outside the Milky Way whose origins are also unknown.
A new theory suggests that the two enigmatic signals could come from a single cosmic source: highly active and magnetized neutron stars called magnetars. If true, that could fill in the details of how fast radio bursts, or FRBs, occur. However, finding the “smoking gun” — catching a simultaneous neutrino and radio burst from the same magnetar — will be challenging because such neutrinos would be rare and hard to find, says astrophysicist Brian Metzger of Columbia University. He and his colleagues described the idea in a study posted September 1 at arXiv.org.
Even so, “this paper gives a possible link between what I think are two of the most exciting mysteries in astrophysics,” says astrophysicist Justin Vandenbroucke of the University of Wisconsin–Madison, who hunts for neutrinos but was not involved in the new work.
More than 100 fast radio bursts have been detected, but most are too far away for astronomers to see what drives the blasts of energy. Dozens of possible explanations have been debated, from stellar collisions to supermassive black holes to rotating stellar corpses called pulsars to pulsars orbiting black holes (SN: 1/10/18). Some astronomers have even invoked signals from aliens.
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But in the last few years, magnetars have emerged as a top contender. “We don’t know what the engines are of fast radio bursts, but there’s growing confidence that some fraction of them is coming from flaring magnetars,” Metzger says.
That confidence got a boost in April, when astronomers detected the first radio burst coming from within the Milky Way galaxy (SN: 6/4/20). The burst was close enough — about 30,000 light-years away — that astronomers could trace it back to a young, active magnetar called SGR 1935+2154. “It’s really like a Rosetta stone for understanding FRBs,” Vandenbroucke says.
There are several ways that magnetars could emit the bursts, Metzger says. The blasts of radio waves could come from close to the neutron star’s surface, for example. Or shock waves produced after the magnetar burped out an energetic flare, similar to those emitted by the sun, could create the radio waves.
Only those shock waves would produce neutrinos and fast radio bursts at the same time, Metzger says. Here’s how: Some magnetars emit flares repeatedly, enriching their surroundings with charged particles. Crucially, each flare would excavate some protons from the neutron star’s surface. Other situations could give a magnetar a halo of electrons, but protons would come only from the magnetar itself. If the magnetar has a halo of electrons, adding protons to the mix sets the stage for the double dose of cosmic phenomena.
As the next flare runs into the protons released by the previous flare, it would accelerate protons and electrons in the same direction at the same speeds. This “ordered dance” of electrons could give rise to the fast radio burst by converting the energy of the electrons’ movement into radio waves, Metzger says. And the protons could go through a chain reaction that results in a single high-energy neutrino per proton.
Together with astrophysicists Ke Fang of Stanford University and Ben Margalit of the University of California, Berkeley, Metzger calculated the energies of any neutrinos that would have been produced by the fast radio burst seen in April. The team found those energies matched those that could be detected by the IceCube neutrino observatory in Antarctica.
But IceCube didn’t detect any neutrinos from that magnetar in April, says Vandenbroucke, who has been searching for signs of neutrinos from fast radio bursts in IceCube data since 2016. That’s not surprising, though. Because neutrinos from FRBs are expected to be rare, detecting any will be challenging, and would probably require a particularly bright magnetar flare to be aimed directly at Earth.
Vandenbroucke has made bets with his students on other aspects of their research, but he says he won’t put any money down on whether he’ll see a neutrino from a fast radio burst in his lifetime. “There’s too much uncertainty,” he says.
Still, he’s optimistic. “Even detecting one neutrino from one [fast radio burst] would be a discovery, and it would take only one lucky FRB to produce a detectable neutrino,” he says. More
