Fast radio bursts, or FRBs, are mysterious bursts of radio waves that come from faraway galaxies. We recently presented evidence for this phenomenon in Nature Astronomy.
When two “supermassive” neutron stars collided, they produced a burst of gravitational waves, each of which was the super-dense core of an exploded star. We discovered that when the neutron star collapsed into a black hole two and a half hours later, it produced an FRB.
So we believe. An optical or gamma-ray flash coming from the direction of the fast radio burst, which would either support or disprove our theory, vanished almost four years ago. We might have another chance to verify our assertions in a few months.
From space, brief and powerful
FRBs are extremely powerful radio wave pulses that last about one thousandth of a second. Astronomers have discovered that the majority of FRBs originate from galaxies that are so far away that light takes billions of years to reach us using data from an Australian radio telescope called the Australian Square Kilometer Array Pathfinder (ASKAP). Astronomers have, however, been baffled by the nature of these radio wave bursts ever since their initial discovery in 2007.
An object in our galaxy known as SGR 1935+2154 provides the best clue. It is a magnetar, a type of neutron star, whose magnetic fields are approximately a trillion times stronger than those of a refrigerator magnet. It produced a violent radio wave burst on April 28, 2020, which was less powerful than an FRB.
Astronomers have known for a long time that if two neutron stars, known as a binary, combine to form a black hole, there should also be a burst of radio waves. The two neutron stars will be exceptionally attractive, and dark openings can’t have attractive fields. The idea is that a rapid radio burst occurs when the magnetic fields suddenly disappear when neutron stars merge and collapse into a black hole. Electric fields are generated by shifting magnetic fields, which is how most power plants generate electricity. Also, the enormous change in attractive fields at the hour of breakdown could deliver the extreme electromagnetic fields of a FRB.
The search for the smoking gun
Alexandra Moroianu, a master’s student at the University of Western Australia, looked for merging neutron stars detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States in order to test this concept. The ripples in spacetime caused by the collision of two massive objects, like neutron stars, are the gravitational waves that LIGO is looking for.
An artist’s rendition of a rapid radio burst that reaches Earth after traveling through space Credit: Two binary mergers of neutron stars have been discovered by ESO and M. Kornmesser, CC BY LIGO.
The second, GW190425, was crucial because it took place at a time when a brand-new FRB-hunting telescope called CHIME was also operational. Notwithstanding its novelty, it took Ring two years to deliver its most memorable cluster of information. At the point when it did as such, Moroianu immediately distinguished a quick radio burst called FRB 20190425A, which happened just a short time after GW 190425.
Even though this was exciting, there was a problem: only one of LIGO’s two detectors was working at the time, so it was impossible to know exactly where GW190425 had come from. In fact, there was a exciting, there was a problem: only one of LIGO’s two detectors was working at the time, so it was impossible to know exactly where GW190425 had come from. In fact, there was a 5% chance that this was just a coincidence.
Worse, at the time, Earth blocked the Fermi satellite, which could have detected gamma rays from the merger—the “smoking gun” confirming GW190425’s origin.
It was unlikely to be a coincidence, but the most important tidbit was that FRBs track the total quantity of gas they have passed through. Because high-frequency radio waves travel through gas more quickly than low-frequency ones, the time difference between them tells us how much gas is present.
Since we know the typical gas thickness of the universe, we can relate this gas content to the Macquart connection. Additionally, the distance traveled by FRB 20190425A was nearly identical to that of GW190425. Bingo!
So have we found the wellspring of all FRBs? No. FRBs must still originate from magnetars, as SGR 1935+2154 did, as there are not enough merging neutron stars in the universe to account for the number.
In addition, despite the abundance of evidence, there is still a one in 200 chance that this is all a huge coincidence. However, LIGO, Virgo, and KAGRA, two additional gravitational wave detectors, will reactivate in May of this year, and they will be more sensitive than ever. CHIME and other radio telescopes, on the other hand, are prepared to immediately detect any FRBs that result from neutron star mergers.
We might find out in a few months if we have made a significant breakthrough or if it was just a fluke.
More information: Alexandra Moroianu et al, An assessment of the association between a fast radio burst and binary neutron star merger, Nature Astronomy (2023). DOI: 10.1038/s41550-023-01917-x