Sometimes we receive strange, ghostly radio signals from outer space, which astronomers have yet to conclusively explain.
Partially because they disappear almost as soon as they
arrive.
The ephemera known as fast radio bursts (FRBs) are among the
Universe's most mysterious occurrences, even more eerie than the supposed
little green aliens with a penchant for physical probes.
If they were related, that would neatly solve two major
mysteries.
But FRBs are bursts of radio waves from the distant cosmos,
generally lasting for tiny fractions of a second (i.e., fast).
Their transience, and the fact that they randomly appear
across the sky, have made it difficult to pin down their origins for nearly two
decades since their discovery in 2007.
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Fast radio bursts arrive from all positions in the sky. |
But we just got a huge clue: Astronomers have reported the
most distant FRB yet, designated FRB 20240304B, a burst of ancient energy that
was emitted in one-thousandth of a second and has traveled through 80 percent
of the Universe's history to meet us.
"What makes fast radio bursts interesting is that we
don't know what generates them," explains lead author Manisha Caleb, a
radio astronomer and astrophysicist at the University of Sydney.
"We have theories for what objects produce them, but we
don't have conclusive proof."
So, who's the cosmic culprit in this specific case? Possibly
a magnetar: a type of neutron star on steroids that packs the most powerful
magnetic fields in the Universe and squeezes the mass of a whole Sun into a
magnetic marble the size of San Francisco Bay.
Yet here's another conundrum: despite their incomprehensibly intense origins, and their ability to release 500 million Suns' worth of energy in thousandths of a second, they may be light(er)weights compared to other stellar outbursts, which has led to a sampler platter of possibilities and added to the difficulty of their conclusive identification:
"We observationalists can only kill models, never prove them. So, when you have 100 to kill, it takes a fair bit of time." – astronomer and astrophysicist J. Xavier Prochaska
"Fundamentally, it is because the energies involved are
modest. They are extreme for radio wave emission, but nothing like the energy
in a supernova or gamma-ray burst," J. Xavier Prochaska, an astronomer and
astrophysicist at the University of California, Santa Cruz, told.
"As such, theorists have devised nearly 100 models to
generate them.
"We observationalists can only kill models, never prove
them. So, when you have 100 to kill, it takes a fair bit of time."
In the study, published in the journal Science, a massive,
international collaboration of astronomers used multiple observatories to
explore this mysterious FRB emanating from 'cosmic noon,' an era only 3 billion
years after the Big Bang when Universal star formation was at its peak.
The astronomers identified the FRB in March 2024 using
MeerKAT, a 64-antenna radio telescope in South Africa's Meerkat National Park.
They then pointed two ultra-sensitive ground telescopes at
the corresponding patch of sky to see its host galaxy and discovered… darkness.
They looked as deep as they could "but there was simply
nothing to see," Prochaska says.
So they called in the heavy orbital artillery (just a
metaphor, for now), the JWST and its exceptional infrared sensitivity.
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The JWST was able to detect the host galaxy of FRB
20240304B, as shown by the inset. |
And if the researchers suspected a monster, they found a mouse:
"We thought it would be a big, nicely formed galaxy with lots of stars, and instead it was a little dwarf galaxy, although it was actively forming stars," says Caleb.
Altogether, this dwarf galaxy is 1,000 times less massivethan expected.
It's also exceedingly young, with 90 percent of its stellar
mass having formed in the past 30 million years.
This helped the astronomers favor a magnetar as the
potential source of the historic FRB.
Another prominent FRB scenario involves the collision of two
neutron stars, a cataclysmic crash that yields a black hole or an extra-heavy
neutron star, plus a bunch of literal gold.
Yet neutron stars dance around each other for billions of
years before colliding, like a still-shy couple on a second date, so it
wouldn't make sense in a young galaxy.
Magnetars, however, can form and release their magnetic
energy in just millions of years. Especially in a metal-poor galaxy like this
that breeds massive stars.
Also, like good guests, FRBs come bearing gifts of the
astrophysical kind.
Even though light travels at the speed of itself, the
ultimate breakneck physical limit, it only does so in a vacuum.
And as this FRB passed through the cold, ionized
(electrically charged) gas that permeates the cosmos, its lower-frequency waves
were slowed more than its higher ones, so they arrived later.
Turbulent gas, most likely in its home galaxy, also
scattered the signal, smearing it out over several milliseconds
FRBs thus act as an astronomical flashlight, and this
particular one has had its signal warped by two cosmic-scale structures as it
traversed most of the Universe to meet us.
In doing so, it helped illuminate an unknown galaxy group
along the way, located about 3.5 billion light-years away, while also bearing
the ionic imprint of the Virgo Cluster, the nearest such structure to Earth,
only around 54 million light-years distant.
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The different wavelengths emitted by the galaxy, such as
those of oxygen and hydrogen, allowed astronomers to determine redshift and
therefore trace the galaxy back to cosmic noon. |
Finally, since we only know of about 100 FRBs whose host
galaxies have been confirmed, the researchers note, and most are billions of
light-years more local, this wasn't just the cherry atop the celestial sundae,
but a whole other sundae on top of the sundae.
"Because we have scant understanding of what generates
FRBs, we could only hope that they might exist at such early times,"
Prochaska told.
"Now, we have proven it and this will drive new,
dedicated studies to find others – and search even greater distances!"