Two colliding neutron stars – each with the superdense core of an exploding star – combined to form a “supermassive” neutron star, causing an explosion of gravitational waves. We found that they produced an FRB when a neutron star collapsed into a black hole two and a half hours later. At least we think so. The most important piece of evidence that could prove or disprove our theory—an optical or gamma-ray burst from the fast radio burst direction—disappeared about four years ago. In a few months, we may have another chance to find out if we’re right.

short and strong

FRBs are incredibly powerful pulses of radio waves from space that last for a thousandth of a second. Using data from a radio telescope in Australia, the Australian Square Kilometer Array Pathfinder (ASKAP), astronomers have discovered that most FRBs come from galaxies so distant that it would take billions of years for light to reach us. But what causes these radio bursts has baffled astronomers since they were first detected in 2007.

The best clue comes from an object in our galaxy known as SGR 1935+2154. It’s a magnetar, a neutron star with magnetic fields about a trillion times stronger than a fridge magnet. On April 28, 2020, it produced a powerful burst of radio waves, similar to the FRB, but less powerful.

Astronomers have long predicted that two neutron stars merging to form a black hole must also produce a burst of radio waves. Two neutron stars are highly magnetic, and black holes cannot have magnetic fields. The idea is that the sudden disappearance of magnetic fields creates a fast radio burst when neutron stars merge into a black hole. Changing magnetic fields create electric fields; most power plants generate electricity this way. The large change in magnetic fields during collapse can cause FRBs to produce intense electromagnetic fields.

Finding the smoking gun

To test this idea, Alexandra Moroianou, a graduate student at the University of Western Australia, looked for merging neutron stars detected by the Laser Interferometric Gravitational-Wave Observatory (LIGO) in the USA. The gravitational waves LIGO is looking for are ripples in space-time produced by the collision of two massive objects, such as neutron stars.

LIGO found the merger of two binary neutron stars. More importantly, the second, known as GW190425, occurred while a new FRB hunting telescope called CHIME was also operating. However, the new one took two years for CHIME to publish its first batch of data. In doing so, Moroianu detected a fast radio burst called FRB 20190425A, which occurred just two and a half hours after GW190425.

As exciting as this was, there was a problem: Only one of LIGO’s two detectors was working at the time, so it was very unclear where exactly GW190425 came from. Actually, the probability of it being a coincidence was 5%. Worse, the Fermi satellite, which may have detected gamma rays from the merger – the “smoking gun” that confirmed the origin of GW190425 – was blocked by Earth at the time.

It’s no coincidence

The critical clue, however, was that the FRBs tracked the total amount of gas they passed through. We know this because high-frequency radio waves travel faster through gas than low-frequency waves, so the time difference between them tells us the amount of gas. Since we know the average gas density in the universe, we can relate this gas content to distance, known as the Macquart ratio. And the distance traveled by the FRB 20190425A was almost perfectly matched to the distance to the GW190425. Bingo!

But have we discovered the source of all the FRBs? NO. There aren’t enough merging neutron stars in the universe to explain the number of FRBs – some still have to come from magnetars, as with SGR 1935+2154. And despite all the evidence, there’s a one in 200 chance that it’s all just a big coincidence. However, LIGO and two other gravitational wave detectors Virgo and KAGRA will return this May and will be more sensitive than ever, while CHIME and other radio telescopes are ready to instantly detect any FRBs from merging neutron stars.