Fast Radio Burst Detected After Gravitational Wave Event
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.
The cyan spot represents FRB20190425A. The red-orange
regions represent the part of the sky from which GW20190425 could have emerged.
(Moroianu et al., Nature Astronomy, 2023) |
GW20190425 was observed by LIGO on 25 April 2019 at 18:18:05
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.