Gravitational Waves From Colliding Neutron Stars Matched to a Fast Radio Burst

Fast Radio Bursts (FRBs) were first detected in 2007 (the Lorimer Burst) and have remained one of the most mysterious astronomical phenomena ever since. These bright radio pulses generally last a few milliseconds and are never heard from again (except in the rare case of Repeating FRBs). And then you have Gravitational Waves (GW), a phenomenon predicted by General Relativity that was first detected on September 14th, 2015. Together, these two phenomena have led to a revolution in astronomy where events are detected regularly and provide fresh insight into other cosmic mysteries.

In a new study led by the Australian Research Council Centre of Excellence for Gravitational Wave Discovery (OzGrav), an Australian-American team of researchers has revealed that FRBs and GWs may be connected. According to their study, which recently appeared in the journal Nature Astronomy, the team noted a potential coincidence between a binary neutron star merger and a bright non-repeating FRB. If confirmed, their results could confirm what astronomers have expected for some time – that FRBs are caused by a variety of astronomical events.

The research team included physicists from OzGrav, the University of Western Australia, the International Centre for Radio Astronomy Research (ICRAR) at Curtin University, and the Nevada Center for Astrophysics (NCfA) at the University of Nevada. The study was led by Alexandra Moroianu, a postgraduate student from UWA’s School of Physics, Mathematics, and Computing, who worked with researchers at OzGrav, ICRAR, and NCfA to study a GW event that happened to coincide with an FRB (a very unlikely coincidence).

The possible causes of FRBs have been debated since they were first detected, with candidates ranging from black holes, neutron stars, and magnetars to possible extraterrestrial transmissions. To date, over 1000 FRBs have been detected thanks to dedicated radio telescopes, like the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Australian Square Kilometre Array Pathfinder (ASKAP). Over time, young magnetars became the most favored candidate by astrophysicists, though recent observations have bolstered the idea that FRBs could have a variety of progenitors.

This includes mergers between neutron stars in compact binary systems. Astronomers have long predicted that these mergers. Clancy W. James, an astrophysicist with ICRAR and co-author of the study, explained in a recent article that appeared in The Conversation:

“Astronomers have long predicted that two neutron stars – a binary – merging to produce a 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 vanishing of magnetic fields when the neutron stars merge and collapse to a black hole produces a fast radio burst. Changing magnetic fields produce electric fields – it’s how most power stations produce electricity. And the huge change in magnetic fields at the time of collapse could produce the intense electromagnetic fields of an FRB.”

To test this theory, Moroianu and her colleagues examined GW190425, a GW event detected on April 25th, 2019, by the Laser Interferometer Gravitational-wave Observatory (LIGO), the Virgo Collaboration, and CHIME. This event was only the second time astronomers detected GWs caused by the inspiral of two non-spinning neutron stars (BNS). Based on the signal properties, the LIGO team estimated that these stars were 1.72 and 1.63 times as massive as the Sun and formed a “supramassive neutron star.”

Using CHIME data first released two years after the event, Moroianu identified a non-repeating fast radio burst (FRB 20190425A), which occurred only two and a half hours after GW190425 and originated from the same spot in the sky. However, confirming that the two were related was rather challenging since one of LIGO’s detectors picked up the GW event (LIGO Livingston). In addition, NASA’s Fermi Gamma-ray Space Telescope was blocked by Earth at the time, which prevented the detection of gamma rays (which would have confirmed that the two events were related).

Nevertheless, the team was able to determine the FRB’s distance by tracing the amount of gas it passed through. This is characteristic of fast radio bursts, where high-frequency radio waves travel through the interstellar medium (ISM) faster than frequency waves. “Because we know the average gas density of the universe, we can relate this gas content to distance, which is known as the Macquart relation,” James added. “And the distance traveled by FRB 20190425A was a near-perfect match for the distance to GW190425. Bingo!”

The authors acknowledge that this coincidence does not prove that FRBs result from neutron star mergers, but it does lend credence to the theory that BNSs could also be a progenitor. Despite the evidence they provide, they also estimate the odds against the two signals being caused by the same event are about 1 in 200. What is needed, at this point, is to find additional examples of FRBs and GW events coinciding. The odds of detecting such events will improve considerably when the recently-upgraded Virgo and Kamioka Gravitational Wave Detector (KAGRA) come back online this May.

With their improved sensitivity, these observatories and their LIGO counterparts (LIGO Hanford and LIGO Livingston) are expected to detect thousands of events in the coming decades. Meanwhile, CHIME, the SKA, and other FRB detectors continue to exponentially increase the number of recorded FRB events, providing a robust basis for comparison. However, as James indicated, we may not have to wait long before more evidence is found:

“The key piece of evidence that would confirm or refute our theory – an optical or gamma-ray flash coming from the direction of the fast radio burst – vanished almost four years ago. In a few months, we might get another chance to find out if we are correct. In a few months, we may find out if we’ve made a key breakthrough – or if it was just a flash in the pan.”

Further Reading: The Conversation