By Stephen Beech
A mystery “alien” signal from outer space has been tracked down to its source.
First discovered in 2007, fast radio bursts, or FRBs, last only a millisecond and can carry an enormous amount of energy – enough to briefly outshine entire galaxies.
Some experts have suggested they may be from an extraterrestrial lifeform trying to contact Earth.
The exact cause and origins of FRBs have remained unconfirmed.
Since the first one was discovered, astronomers have detected thousands of FRBs, whose locations range from within our own galaxy to as far as eight billion light-years away.
Exactly how the cosmic radio flares are launched is a highly contested unknown.
Now, astronomers at the Massachusetts Institute of Technology (MIT) in the United States have pinned down the origins of at least one FRB using a new technique that could do the same for others.
For the study, published in the journal Nature, the MIT team focused on FRB 20221022A – a previously discovered FRB that was detected from a galaxy about 200 million light-years away.
They zeroed in further to determine the precise location of the radio signal by analysing its “scintillation,” similar to how stars twinkle in the night sky.
The scientists studied changes in the FRB’s brightness and determined that the burst must have originated from the immediate vicinity of its source, rather than much further out, as some models have predicted.
The team estimates that FRB 20221022A exploded from a region that is extremely close to a rotating neutron star, 10,000 kilometers away at most.
At such close range, scientists say the burst likely emerged from the neutron star’s magnetosphere – a highly magnetic region immediately surrounding the ultracompact star.
The team’s findings provide the first conclusive evidence that a FRB can originate from the magnetosphere, the highly magnetic environment immediately surrounding an extremely compact object.
Study lead author Dr. Kenzie Nimmo, of MIT’s Kavli Institute for Astrophysics and Space Research, said: “In these environments of neutron stars, the magnetic fields are really at the limits of what the universe can produce.
“There’s been a lot of debate about whether this bright radio emission could even escape from that extreme plasma.”
Dr. Kiyoshi Masui, Associate Professor of Physics at MIT, said: “Around these highly magnetic neutron stars, also known as magnetars, atoms can’t exist – they would just get torn apart by the magnetic fields.
“The exciting thing here is, we find that the energy stored in those magnetic fields, close to the source, is twisting and reconfiguring such that it can be released as radio waves that we can see halfway across the universe.”
Detections of FRBs have soared in recent years, due to the Canadian Hydrogen Intensity Mapping Experiment (CHIME).
The radio telescope array comprises four large, stationary receivers that are tuned to detect radio emissions within a range that is highly sensitive to fast radio bursts.
Since 2020, CHIME has detected thousands of FRBs from all over the universe.
To determine where FRBs arise, the MIT team considered scintillation – the effect that occurs when light from a small bright source such as a star, filters through some medium, such as a galaxy’s gas.
As the starlight filters through the gas, it bends in ways that make it appear, to a distant observer, as if the star is twinkling. The smaller or the farther away an object is, the more it twinkles.
The light from larger or closer objects, such as planets in our own solar system, experience less bending, and therefore do not appear to twinkle.
The team reasoned that if they could estimate the degree to which an FRB scintillates, they might determine the relative size of the region from where the FRB originated.
To test their idea, the researchers looked to FRB 20221022A, detected by CHIME in 2022.
The signal lasts about two milliseconds, and is a relatively run-of-the-mill FRB, in terms of its brightness.
However, the team’s collaborators at McGill University in Canada found that FRB 20221022A exhibited one standout property: The light from the burst was highly polarised, with the angle of polarisation tracing a smooth S-shaped curve.
The pattern is interpreted as evidence that the FRB emission site is rotating – a characteristic previously observed in pulsars, which are highly magnetized, rotating neutron stars.
To see a similar polarisation in FRBs was a first, suggesting that the signal may have arisen from the close-in vicinity of a neutron star.
The MIT team realized that if FRB 20221022A originated from close to a neutron star, they should be able to prove it, using scintillation.
Dr. Nimmo and her colleagues analyzed data from CHIME and observed steep variations in brightness that signaled scintillation — in other words, the FRB was twinkling.
They confirmed that there is gas somewhere between the telescope and FRB that is bending and filtering the radio waves.
The team then determined where the gas could be located, confirming that gas within the FRB’s host galaxy was responsible for some of the scintillation observed.
The gas acted as a “natural lens” – allowing the researchers to zoom in on the FRB site and determine that the burst originated from an extremely small region, estimated to be about 10,000 kms wide.
Dr. Nimmo said: “This means that the FRB is probably within hundreds of thousands of kilometres from the sources.
“That’s very close. For comparison, we would expect the signal would be more than tens of millions of kilometers away if it originated from a shockwave, and we would see no scintillation at all.”
Dr. Masui said: “Zooming in to a 10,000-kilometre region, from a distance of 200 million light years, is like being able to measure the width of a DNA helix, which is about two nanometres wide, on the surface of the moon.
“There’s an amazing range of scales involved.”
The findings prove for the first time that FRBs can originate from very close to a neutron star, in highly chaotic magnetic environments.
Dr. Masui added: “These bursts are always happening, and CHIME detects several a day.
“There may be a lot of diversity in how and where they occur, and this scintillation technique will be really useful in helping to disentangle the various physics that drive these bursts.”
