The accretion of neutron stars may have triggered two different types of cosmic signals: ripples in space-time known as gravitational waves and a short flash of energy called a fast radio burst.
One of the three detectors that make up LIGO’s Gravitational-Wave Observatory picked up a signal from a cosmic collision on April 25, 2019. About 2.5 hours later, a fast radio burst detector picked up A signal from the same area of the skyresearchers report March 27 in natural astronomy.
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If bolstered by more observations, the discovery could bolster the theory that mysterious fast radio bursts have multiple origins — and neutron star mergers are one of them.
“We’re 99.5 percent sure” that the two signals came from the same event, says astrophysicist Alexandra Moroyano, who spotted the merger and its aftermath while at the University of Western Australia in Perth. “We want to be 99.999 percent sure.”
Unfortunately, two other LIGO detectors didn’t pick up the signal, so it’s impossible to pinpoint its exact location. “Although it’s not a concrete, solid observation of something that’s been hypothesized for a decade, it’s the first evidence we have,” Moroianu says. “If true… it would be a major breakthrough in fast radio burst science.”
Mysterious radio blasts
Astronomers have detected more than 600 fast radio pulses, or FRBs, since 2007. Despite their frequency, the causes remain a mystery. One of the prime candidates is a highly magnetized neutron star called a magnetar, which could be left over after the explosion of a massive star (SN: 6/4/20). But some FRBs appear to be recurring, while others appear to be one-off events, which indicates that there is more than one way to produce them (SN: 2/7/20).
Theorists wondered if a collision between two neutron stars could trigger a unique FRB, before the collision debris produced a black hole. Such a collapse should also emit gravitational waves (SN: 10/16/17).
Moroianu and colleagues looked at archived data from LIGO and the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, a fast radio burst detector in British Columbia, to see if any of their signals lined up. The team found an association with one candidate: GW190425 and FRB20190425A.
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Although the gravitational wave was only picked up by the LIGO detector in Livingston, Los Angeles, the team detected other signals that suggested the signals were linked. Both the FRB and the gravitational waves came from the same distance, about 370 million light-years from Earth. Gravitational waves from neutron star mergers were the only ones LIGO detected on this observation path, and the FRB was particularly bright. There may have been a gamma-ray burst at the same time, according to satellite data — another aftereffect of a neutron star merger.
“Everything indicates that this is a very interesting combination of signals,” Moroianu says. It’s like watching a crime drama on TV, she says: “You have a lot of evidence that anyone who watches the TV show is going to be like, ‘Oh, I think he did that.'” But this is not enough to convince the court.
Secrets of the neutron star
Despite the uncertainty, the discovery has exciting implications, says astrophysicist Alessandra Corsi of Texas Tech University in Lubbock. One of them is the possibility of two neutron stars merging into a single massive neutron star without immediately collapsing into a black hole. “There is this fuzzy dividing line between a neutron star and what a black hole is,” says Corsi, who was not involved in the new work.
In 2013, astrophysicist Bing Zhang of the University of Nevada at Las Vegas suggested that The crash of a neutron star can create a supermassive neutron star It teetered on the brink of stability for a few hours before collapsing into a black hole. In this case, the resulting FRB will be delayed – just as in the case of 2019.
The most massive neutron star observed so far has a mass of about 2.35 times the mass of the Sun, but theorists believe it can grow up to be about three times the mass of the Sun without collapsing (SN: 7/22/22). According to Moroianu and his colleagues, the neutron star that would have resulted from the collision in 2019 would be 3.4 solar masses.
“Something like this, especially if confirmed with more observations, would certainly tell us something about how neutron matter behaves,” Corsi says. “The nice thing about this is that we hope to test this in the future.”
The next LIGO run is expected to start in May. Corsi is optimistic that more coincidences between gravitational waves and FRBs will emerge, now that researchers know how to look for them. “There must be a bright future ahead of us,” she says.