Every so often, a strange signal from outer space hits our detectors here on Earth.
Known as fast radio bursts (FRBs), these signals are extremely short, just milliseconds in duration, and are detected only in radio wavelengths.
Yet in those milliseconds, and in those wavelengths, they can discharge as much energy as 500 million Suns – and most of them have never been detected again.
What they are, and how they are generated, is something of a baffling mystery. But a new discovery could point to a previously unknown mechanism producing these powerful bursts of radiation.
On the 25th of April in 2019, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) recorded a bright, non-repeating fast radio burst ( FRB).
Just 2.5 hours earlier, the Laser Interferometer Gravitational-Wave Observatory (LIGO) recorded a gravitational wave event, the collision as a binary neutron star reached the inevitable conclusion of its decaying orbit.
The FRB's location in the sky fell within the credible region of the gravitational wave event, and from a similar distance. The chance that the two events were unrelated, a team of astronomers led by Alexandra Moroianu of the University of Western Australia has determined, is extremely small.
FRBs are extremely enigmatic; only a few of them repeat, and the one-off nature of the vast majority makes them extraordinarily difficult to study.
Their detection used to be chance only; you had to be studying the right patch of the sky at the right time to catch one. All-sky surveys, however, have increased the number of detections to over 600.
A breakthrough came in 2020: for the first time, an FRB was detected coming from within the Milky Way galaxy. It was traced to a type of neutron star called a magnetar, whose insanely powerful external magnetic field fights against the inward pull of gravity, causing the star to occasionally quake and flare.
But while misbehaving magnetars present one explanation, we don't know if that's the whole picture. FRBs vary quite a bit, and it's likely that there's more than one mechanism that can produce them.
There are several theories that predict an association between FRBs and gravitational waves, particularly if neutron stars are involved, either during or following the gravitational wave detection.
So Moroianu and her colleagues went looking in catalogs. The CHIME catalog of observations from July 2018 to July 2019 overlapped with the LIGO-Virgo observation run, for a total of 171 FRB events.
The researchers cross-referenced these events with the GWTC-2 catalog, looking for FRB events that occurred temporally close to gravitational wave detections, within the patch of the sky identified by LIGO.
And they got a very palpable hit.
GW20190425 was observed by LIGO on 25 April 2019 at 08:18:05 UTC. The absence of a detection by the Virgo detector helped constrain the region from which the detection had emerged. Its estimated distance was around 520 million light-years away, generated by a merger between two neutron stars.
FRB20190425A was detected the same day, at 10:46:33 UTC, within the range of sky LIGO had laid out as a plausible source of the neutron star merger, and with an upper distance limit of 590 million light-years.
This, they found, would be an uncanny coincidence if the two were unrelated. The probability of the two events occurring at the distances given, the timeframe of detection, and within the region of space defined by LIGO was just 0.00019, the researchers calculated.
The two events likely emerged from a galaxy called UGC 10667, but the mechanism that produced the FRB might take a little more analysis.
For now, the team believes that the burst was caused by a blitzar, a mechanism proposed for FRBs nearly a decade ago. This is when a neutron star too massive to remain supported by degeneracy pressure collapses into a black hole when its spin slows – the only thing that was preventing this collapse.
"Although we cannot definitively assign the potential GW-FRB association to a single theory, it is consistent with the GW, short gamma-ray burst (sGRB) and FRB association theory that invokes the collapse of a post-binary neutron star-merger magnetar," the researchers write.
"The FRB generation mechanism is the so-called blitzar mechanism, which has been confirmed through numerical simulations. Within this scenario, the 2.5-hour delay time between the FRB and the GW event is the survival time of the supramassive neutron star before collapsing into a black hole, which is consistent with the expected range of the delay timescale for a supramassive magnetar from both theory and observational data."
The masses of the neutron stars of GW20190425 were significantly higher than most neutron star binaries detected in the Milky Way. These lower mass binaries would produce more stable heavyweight neutron stars after they merge, which could survive a long time and repeatedly spit out FRBs, thus explaining the few repeating FRB sources.
Whether or not the two events were linked remains to be confirmed, but one thing is certain: the estimated rate of binary neutron star mergers is far, far lower than the rate at which FRBs like FRB190425A are detected. So this potential mechanism cannot, alone, account for the mysterious signals that sputter across the radio sky.
Further investigation is still warranted. But it's tremendously exciting that we seem to be closing in on some answers.
The research has been published in Nature Astronomy.