Many of the brightest, weirdest phenomena in space come from cataclysmic events like explosions or collisions. But many fast radio bursts (FRBs), one of the most mysterious space signals we’ve seen, must not. That might mean that they are all part of a class of FRB that we previously thought might be rare.
FRBs are milliseconds-long bursts of powerful radio waves that come from the depths of space. They have been attributed to many different sources, from neutron star mergers to alien spaceships, but no explanations have definitively fit yet.
Most of the FRBs we’ve spotted appear only once, but three appear to repeat, sending multiple blasts of radio waves through space. Those three cannot come from cataclysmic events like neutron star collisions or supernovae that destroy their progenitors.
Now, Vikram Ravi at the California Institute of Technology has calculated that the rest probably don’t either. He used a few of the closest non-repeating FRBs we’ve seen to calculate a lower limit on how often they occur, and compared that rate to the rates of cataclysmic events in the nearby universe.
“The rate of FRBs appears to be higher than the rate of anything we can really think of that can make an FRB just once,” Ravi says. That means that a single type of explosion or collision cannot account for all the FRBs.
It’s possible that all of the proposals for cataclysmic sources are simultaneously correct, he says, but perhaps more likely is that most – or even all – FRBs are actually repeaters. That way, each source produces multiple bursts over its lifespan and we don’t need as many sources. We may not be detecting all of the repeated bursts because they are slower or dimmer than the repeating sources that we have seen.
“I don’t think we can rule out that there are multiple classes of things that go boom in the radio sky,” says Victoria Kaspi at McGill University in Montreal, Canada. “We have suspected and have some evidence that there are multiple classes, and what fraction belong to each class is unknown.” Because none of our models thus far quite fit, she says it’s also possible the FRBs are formed by events we’ve never seen or considered before.
“We need to say very specifically what sorts of galaxies FRBs come from and where in those galaxies they come from,” says Ravi – just three of them have been localised so far. “If we’re doing our jobs right, we should be able to figure it out in the next 5 years.”
Journal reference: Nature Astronomy, DOI: 10.1038/s41550-019-0831-y
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