For the first time, astronomers have linked a mysterious fast radio burst with gravitational waves

By Clancy William James, Senior Lecturer (Astronomy and Astroparticle Physics), Curtin University


We have just published evidence in Nature Astronomy for what can produce mysterious bursts of radio waves emanating from distant galaxies, known as fast radio bursts or FRB.

Two that collide neutron stars – each the super-dense core of an exploded star – produced a burst of gravitational waves as they merged into a “supermassive” neutron star. We discovered that two and a half hours later they produced an FRB as the neutron star collapsed into a black hole.

Or so we think. The main piece of evidence that would confirm or disprove our theory—an optical flash, or gamma-ray burst, coming from the direction of the fast radio burst—disappeared nearly four years ago. In a few months we may get another chance to find out if we are right.

Short and powerful

FRBs are incredibly powerful pulses of radio waves from space that last about a thousandth of a second. Using data from a radio telescope in Australia, the Australian Square Kilometer Array Pathfinder (ASH), astronomers have found that most FRBs come from galaxies that are so far away that light takes billions of years to reach us. But what produces these radio wave bursts has puzzled astronomers ever since a first discovery during 2007.

The best clue comes from an object in our galaxy called SGR 1935+2154. It is a magnets, which is a neutron star with a magnetic field about a trillion times stronger than a refrigerator magnet. On April 28, 2020, it produced a violent bursts of radio waves – similar to an FRB, but less powerful.

Read more:
A brief history: what we know so far about fast radio spreading across the universe

Astronomers have long predicted that two neutron stars – a binary – merge to produce one black hole should also produce a burst of radio waves. The two neutron stars will be highly magnetic, and black holes cannot have magnetic fields. The idea is the sudden disappearance of magnetic fields when neutron stars merge and collapse into a black hole producing a rapid radio burst. Changing magnetic fields produce electric fields – this is how most power plants produce electricity. And the enormous change in magnetic field at the time of collapse can produce the intense electromagnetic fields of an FRB.

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Artist's impression of a fast radio burst traveling through space and reaching Earth.
ESO/M. Grain fairs, CC BY

The search for the smoking gun

To test this idea, Alexandra Moroianu, a master's student at the University of Western Australia, looked for merged neutron stars detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S. The gravitational waves LIGO is looking for are ripples in spacetime, produced by collisions between two massive objects, such as neutron stars.

LIGO has found two binary neutron star mergers. Crucially, the other, known as GW190425occurred when a new FRB chase telescope called CHIME was also in operation. But since it was new, it took CHIME two years to release its first batch of data. As it did so, Moroianu quickly identified a rapid radio burst being called FRB 20190425A which occurred just two and a half hours after GW190425.

As exciting as this was, there was one problem – only one of LIGO's two detectors was working at the time, making very uncertain where exactly GW190425 had come from. In fact, there was a 5% chance that this could just be a coincidence.

Worse, that Fermi satellite, which could have detected gamma rays from the merger – the “smoking gun” confirming the origin of GW190425 – was blocked by the earth Right then.

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CHIME, the Canadian Hydrogen Intensity Mapping Experiment, has proven to be uniquely suited to detecting FRBs.
Andre Renard/Dunlap Institute/CHIME Collaboration

It is hardly a coincidence

The critical clue, however, was that the FRB tracks the total amount of gas that they have passed through. We know this because high-frequency radio waves travel faster through the gas than low-frequency waves, so the time difference between them tells us the amount of gas.

Because we know the average gas density of the universewe can relate this gas content to distance, which is known as Macquart ratio. And the distance traveled by FRB 20190425A was an almost perfect match for the distance to GW190425. Bingo!

So have we discovered the source of all FRBs? No. There are not enough merged neutron stars in the universe to account for the number of FRBs – some must still come from magnetars, as SGR 1935+2154 did.

And even with all the evidence, there's still a 200 chance that this could be a giant coincidence. But LIGO and two other gravitational wave detectors, Virgo and KAGRAcomes turn on again in May this year, and be more sensitive than ever, while CHIME and other radio telescopes is ready to immediately detect all FRBs from neutron star mergers.

In a few months, we can find out if we have made an important breakthrough – or if it was just a flash in the pan.

Clancy W. James would like to thank Alexandra Moroianu, lead author of the study; his co-authors, Linqing Wen, Fiona Panther, Manoj Kovalem (University of Western Australia), Bing Zhang and Shunke Ai (University of Nevada); and his late mentor, Jean-Pierre Macquart, who experimentally verified the relationship between gas and distance, which is now named after him.

The conversation

Clancy William James receives funding from the Australian Research Council.

Originally published in The conversation.

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