An accreting neutron star could emit two different types of cosmic signals: ripples in space-time known as gravitational waves and a short burst of energy called a fast radio burst.
One of the three detectors that make up the LIGO Gravitational Wave Observatory recorded a signal from a cosmic collision on April 25, 2019. About 2.5 hours later, the fast radio burst detector recorded a signal from the same region of the sky, the researchers report. March 27 in Nature Astronomy .
If this discovery is supported by further observations, it could support the theory that mysterious fast radio bursts have multiple sources, and neutron star mergers are one of them.
“We are 99.5 percent sure” that the two signals came from the same event, says astrophysicist Alexandra Moroianu, who observed the merger and its aftermath while working at the University of Western Australia in Perth. “We want to be 99.999% sure.”
Unfortunately, LIGO’s other two detectors did not pick up the signal, so its exact location cannot be determined. “While this is not a concrete, substantial observation of what has been theorized for a decade, it is the first evidence we have,” Moroianu says. “If this is true … it will be a big boom in the science of fast radio bursts.”
Mysterious radio bursts
Since 2007, astronomers have observed more than 600 Fast Radio Bursts, or FRBs. Despite their frequency, the causes remain a mystery. One leading candidate is a highly magnetized neutron star called a magnetar, which may be left behind after a massive star explodes. But some FRBs appear to repeat, while others are one-off, suggesting that there is more than one way to create them.
Theorists have wondered whether the collision of two neutron stars could trigger a singular FRB before the debris from the collision creates a black hole. Such a breach should also emit gravitational waves.
Moroianu and his colleagues searched archived data from LIGO and the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, a fast radio flare detector in British Columbia, to see if any of their signals matched. The team found one possible pair: GW190425 and FRB20190425A.
Although the gravitational wave was only detected by the LIGO detector in Livingston, Louisiana, the team noticed other signs indicating that the signals are related. FRBs and gravitational waves came from the same distance, about 370 million light years from Earth. The gravitational waves came from the only neutron star merger LIGO saw during those observations, and the FRB was particularly bright. According to satellite data, there may even have been a gamma-ray burst at the same time, another effect of the neutron star merger.
“Everything points to this being a very interesting combination of signals,” Moroianu says. She says it’s like watching a crime drama on TV: “You have so much evidence that anyone who watches the TV show will say, ‘Oh, I think he did it.’ But this is not enough to convince the court.”
The secrets of the neutron star
Despite the uncertainty, the discovery has exciting implications, says astrophysicist Alessandra Corsi of Texas Tech University in Lubbock. One is that two neutron stars can merge into one supermassive neutron star without immediately collapsing into a black hole. “There is a fine line between what is a neutron star and what is a black hole,” says Corsi, who was not involved in the new work.
In 2013, astrophysicist Bing Zhang of the University of Nevada, Las Vegas suggested that the collapse of a neutron star could create a supermassive neutron star that hovers on the edge of stability for several hours before collapsing into a black hole. In that case, the final FRB will be delayed—as was the case in 2019.
The most massive neutron star observed is about 2.35 times the mass of the Sun, but theorists believe it could grow to about three solar masses without collapsing. According to calculations by Moroyan and his colleagues, the neutron star that could have resulted from the collision in 2019 would have 3.4 solar masses.
“Something like this, especially if it’s confirmed by additional observations, it will definitely tell us something about how neutron matter behaves,” says Corsi. “The great thing about it is that we hope to test it in the future.”
The next LIGO launch is expected to begin in May. Corsi is optimistic that more coincidences between gravitational waves and FRBs will emerge now that researchers know to look for them. “We should have a bright future ahead of us,” she says.