The mystery of FRB’s deepens: many of the brightest, weirdest phenomena in space come from cataclysmic events like explosions or collisions. But many fast radio bursts (FRBs), milliseconds-long bursts of powerful radio waves that come from the depths of space have been attributed to many different sources, from neutron star mergers to alien spaceships. But so far, no explanation seems plausible.
Most of the FRBs detected 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 objects of origin.
Enter Vikram Ravi at the California Institute of Technology, who focus is on exploring and understanding the ephemeral, unseen universe, and has calculated that the rest probably don’t either. He used the closest non-repeating FRBs detected 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, which 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.”
“Finding the locations of the one-off FRBs is challenging because it requires a radio telescope that can both discover these extremely short events and locate them with the resolving power of a mile-wide radio dish,” says Vikram Ravi, a new assistant professor of astronomy who works with the radio telescopes at Caltech’s Owens Valley Radio Observatory (OVRO), which is situated east of the Sierra Nevada mountains in California.
“At OVRO, we built a new array of ten 4.5-meter dishes that collectively act like a mile-wide dish to cover an area on the sky the size of 150 full moons,” he says. “To do this, a powerful digital system ingests and processes an amount of data equivalent to a DVD every second.”
The new OVRO instrument is called the Deep Synoptic Array-10, with the “10” referring to the number of dishes. This array serves as a stepping stone for the planned Deep Synoptic Array (DSA), funded by the National Science Foundation (NSF), which, when completed by 2021, will ultimately consist of 110 radio dishes.
“The DSA is expected to discover and localize more than 100 FRBs per year,” says Richard Barvainis, program director at the NSF for the Mid-Scale Innovations Program, which is funding the construction of the DSA. “Astronomers have been chasing FRBs for a decade now, and we’re finally drawing a bead on them with new instruments like DSA-10 and, eventually, the full DSA. Now we have a chance of figuring out just what these exotic objects might be.”
“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 no model quite fits, she says it’s 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 localized so far. “If we’re doing our jobs right, we should be able to figure it out in the next 5 years.”