Ask Dr. SETI ®
by Seth Shostak
Senior Astronomer, SETI Institute
From The SETI Institute's Journey e-newsletter,
9 September 2019, used by permission
Just when you think you’ve cataloged all the beasts of the cosmos, a new one howls to us from the celestial savanna. Fast radio bursts are now one of the hottest topics in astronomy. In less time than an eye blink, these mysterious objects can release enough energy to power the world for three centuries. And the race is on to figure out what the heck they are.
Last month, a consortium of five dozen astronomers reported the discovery of eight new bursts that may lead to an answer. The objects were found with the Canadian Hydrogen Intensity Mapping Experiment, or CHIME. This unusual-looking radio telescope, about the size of a football field, consists of four metal mesh cylinders — like skateboard half-pipes — that collect and focus incoming radio waves. CHIME is in a sparsely populated, mountainous region of British Columbia about 30 miles north of the U.S. border.
While CHIME is leading the pack today in discovering radio bursts, the first such burst was found a dozen years ago by a West Virginia University astronomer sitting at his desk in Morgantown. Duncan Lorimer was combing through data obtained from a radio telescope in Parkes, Australia — half a world away — when he noticed a short burp of static, the kind of signal you’d produce by firing up a transmitter and then turning it off a few milliseconds later.
Dozens more FRBs were found thereafter. But all were like the first: “one-offs” that briefly belched radio waves into space and then were gone. That made it impossible to zero in on their location. It’s a bit like hearing a momentary squeak from under your car’s hood. If you hear it only once, chances are poor that you’ll ever pin down its location or its cause.
But in 2012, the Arecibo radio telescope in Puerto Rico detected an FRB that eventually changed this frustrating situation. A few years after its discovery, this object was observed to hiccup again … and again, every few weeks or so. It was like a squeak that repeats, giving you the chance to raise the hood and pinpoint the source. In the case of 121102 (as it’s lyrically named), astronomers used large radio telescope arrays — which are good for pinpointing sources on the sky — to learn that this FRB was in a nondescript galaxy 3 billion light-years away.
That phenomenal distance — 5 trillion times farther than Pluto — implies that whatever does the bursting is more energetic than a passel of puppies. Additionally, the fact that all FRBs are of short duration means that whatever’s causing them is pretty small. Think of it this way: If a small group of people standing close together all say “boo” at the same time, then it’s a pretty short sound. But if you’re dealing with a large mob, the sound from the back takes a while to reach you, so the “boo” becomes a long wail.
Because FRBs last only a thousandth of a second or so, any object producing them couldn’t be much bigger than 200 miles across — the distance radio waves can cover in that time. This suggests that the source is considerably smaller than an ordinary star: it might be a collapsed stellar corpse — the compact remains of a star that has exhausted its nuclear fuel — or possibly a black hole that's swallowing something or undergoing a collision.
While collisions might explain some of the FRBs, they don't make sense for the repeaters. You can’t back up a couple of tangling black holes and have them give a repeat performance. And keep in mind that it’s entirely possible that the one-hit wonders are actually repeaters whose encores haven’t yet been heard. All FRBs might have the hiccups.
There are literally dozens of celestial objects that might be causing FRBs, ranging from prosaic things like souped-up supernovae — large stars that blow up at the ends of their lives — to weird things like magnetars — collapsed stellar corpses awash in strong magnetic fields. Some of the suggested explanations are truly far out and involve fracturing stellar crusts or exotic cracks in spacetime known as cosmic strings.
There are some who suggest that the FRBs might be alien signals, but that really doesn’t make sense. The sources are spread all over intergalactic space, and arranging cooperative alien behavior when even one-way communication takes many billions of years seems unlikely — to put it gently.
Fact is, we simply don’t know what causes FRBs. But here’s my take: Almost every time astronomers turn up some unexpected new phenomenon in the sky, they’re baffled. Quasars and pulsars were deeply puzzling when first discovered. So researchers adopt a Sherlock Holmes approach and collect clues — which is to say, they find as many examples of the new phenomenon as they can. These observations became grist for the theoreticians — wonky types who like nothing better than to solve nature’s riddles.
Once there’s an abundance of data, the theoreticians can usually cook up the correct explanation pretty quickly. This isn’t always the case, of course. We still don’t know what dark matter and dark energy are, despite decades of observation.
But I’m betting the odds. The new, repeating FRBs uncovered by CHIME are likely game-changers because they’re ripe to be pinpointed and studied in detail. The drama of the FRBs is about to enter its second act, and I dare say that two years from now FRBs will be just another critter in the cosmic zoo.
Exotic and novel, yes, but at least we’ll understand what they are.
Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.
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this page last updated 5 October 2019
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