Last year, we made an intriguing discovery – a radio signal in space that switched on and off every 18 minutes.
Astronomers expect to see some repeating radio signals in
space, but they usually blink on and off much more quickly. The most common
repeating signals come from pulsars, rotating neutron stars that emit energetic
beams like lighthouses, causing them to blink on and off as they rotate towards
and away from the Earth.
Pulsars slow down as they get older, and their pulses become fainter, until eventually they stop producing radio waves altogether. Our unusually slow pulsar could best be explained as a magnetar – a pulsar with exceedingly complex and powerful magnetic fields that could generate radio waves for several months before stopping.
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Pulsars emit powerful beams of radio waves from their poles,
which sweep across our line of sight like a lighthouse. Joeri van Leeuwen |
Unfortunately, we detected the source using data gathered in
2018. By the time we analysed the data and discovered what we thought might be
a magnetar it was 2020, and it was no longer producing radio waves. Without
additional data, we were unable to test our magnetar theory.
Nothing new under the sun
Our Universe is vast, and so far every new phenomenon we’ve
discovered has not been unique. We knew that if we looked again, with
well-designed observations, we had a good chance of finding another long-period
radio source.
So, we used the Murchison Widefield Array radio telescope in
Western Australia to scan our Milky Way galaxy every three nights for several
months.
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A single tile of the hundreds that make up the Murchison
Widefield Array, a radio telescope in outback Western Australia. Natasha
Hurley-Walker |
We didn’t need to wait long. Almost as soon as we started
looking, we found a new source, in a different part of the sky, this time
repeating every 22 minutes.
At last, the moment we had been waiting for. We used every
telescope we could find, across radio, X-ray, and optical light, making as many
observations as possible, assuming it would not be active for long. The pulses
lasted five minutes each, with gaps of 17 minutes between. Our object looked a
lot like a pulsar, but spinning 1,000 times slower.
Hiding in plain sight
The real surprise came when we searched the oldest radio
observations of this part of the sky. The Very Large Array in New Mexico,
United States, has the longest-running archive of data. We found pulses from
the source in data from every year we looked – the oldest one in an observation
made in 1988.
Observing over three decades meant we could precisely time
the pulses. The source is producing them like clockwork, every 1,318.1957
seconds, give or take a tenth of a millisecond.
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Radio images taken with the MeerKAT radio telescope in South
Africa, showing one of the pulses, sped up by a factor of 20. The other nearby
radio sources remain constant. Natasha Hurley-Walker |
According to our current theories, for the source to be
producing radio waves, it should be slowing down. But according to the
observations, it is not.
In our article in Nature, we show that the source lies
“below the death line”, which is the theoretical limit of how neutron stars
generate radio waves; this holds even for quite complex magnetic field models.
Not only that, but if the source is a magnetar, the radio emission should only
be visible for a few months to years – not 33 years and counting.
So when we tried to solve one problem, we accidentally
created another. What are these mysterious repeating radio sources?
What about ET?
Of course, it’s very tempting at this point to reach to
extraterrestrial intelligence as an option. The same thing happened when
pulsars were discovered: astrophysicist Jocelyn Bell Burnell and her
colleagues, who found the first pulsar, nicknamed it “LGM 1”, for “Little Green
Men 1”.
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Observation of pulses from the first pulsar to be
discovered, CP 1919. The chart recorder shows regular deflections every 1.3
seconds. Jocelyn Bell Burnell and Anthony Hewish |
But as soon as Bell and her colleagues made further
detections, they knew it could not be aliens. It would be incredibly unlikely
for so many similar signals to be coming from so many different parts of the
sky.
The pulses, similar to those of our source, contained no
information, just “noise” across all frequencies, just like natural radio
sources. Also, the energy requirements to emit a signal at all frequencies are
staggering: you need to use, well, a neutron star.
While it’s tempting to try to explain a new phenomenon this
way, it’s a bit of a cop out. It doesn’t encourage us to keep thinking,
observing and testing new ideas. I call it the “aliens of the gaps” approach.
Fortunately, this source is still active, so anyone in the
world can observe it. Perhaps with creative follow-up observations, and more
analysis, we’ll be able to solve this new cosmic mystery.