In 2022, astronomers using the Murchison Widefield Array discovered a strange new radio signal
that was arriving every 22 minutes.
The astronomers were no strangers to such repeating signals – they typically come
from pulsars; neutron stars which send intense pulses of light across the universe as they
rotate on their axis.
But as they began to look deeper into records of past observations, they realised this signal
had been arriving at Earth since at least 1988, with remarkable stability – far more
stable than is expected for a pulsar rotating every 22 minutes.
If it was a neutron star, it was unlike any they had seen before.
So where was this signal coming from?
I’m Alex McColgan, and you’re watching Astrum.
Join me today as we grapple with the mystery that lies behind this signal, which will challenge
our understanding of some of the most awe-inspiring objects in our cosmos.
The location of the source, named GPM J1839–10, is roughly 18 thousand light years away.
Its signal arrives as pulses that can last any amount of time between 30 seconds and
5 minutes.
These pulses can appear at any time in a window of just over 6 and a half minutes, which is
centred on 22 minutes after the previous pulse.
To us, this may seem like a great deal of variation.
30 seconds and 5 minutes are very different durations, and the pulse arrival varying by
over 6 minutes doesn’t paint a picture of a very stable source.
But neutron star dynamics can be very complicated, and if the source is indeed a neutron star,
then many factors can affect the duration and arrival times of the pulses that we receive.
Nevertheless, the astronomers were able to spot this signal hiding in data from the last
35 years and used this expanded data set to average out the fluctuations.
They calculated that the source was rotating once every 21 minutes and 58 seconds, as well
as how much it had slowed down.
But to their surprise, they calculated that this rotation period remained unchanged over
the past 35 years, even though it is expected that the source will slow down as it radiates
energy into space.
We can only say for sure that if the source has slowed down, its rotation period could
not have increased by more than 0.28 milliseconds over the 35-year period because otherwise
we would have been able to detect this in our data.
This is an absolutely minuscule amount, and it shows that whatever the object is, it is
spinning with remarkable stability.
This usually isn’t odd for a pulsar.
These are the timekeepers of the universe, the clocks of the cosmos, mechanistically
ticking away with such certainty that we can use them to measure time across vast stretches
of the universe.
However, this level of stability is odd for a pulsar that is rotating so slowly.
To understand what makes it so odd, we need to recap how pulsars work and what makes them
slow down over time.
Pulsars are neutron stars: the leftover cores of dead supergiant stars which, barring black
holes, are the densest objects in the universe.
Planets like our own are made up of atoms, which consist of over 99.999% empty space
due to the vast separation between the electrons and the incredibly dense nucleus they are
whizzing around.
But imagine an entire star made purely out of the neutrons that are found in the nucleus.
No electrons, no protons, no empty space.
Merely a teaspoon of it would have as much mass as 11 times that of the entire human
population - all 8 billion people.
A typical neutron star is around 35% more massive than our Sun, and squeezed into a
sphere that has a diameter about as long as the island of Manhattan.
To call it dense would be an understatement.
For reasons still unknown to astrophysicists, this extreme environment gives rise to an
incredibly strong magnetic field.
What does this have to do with the signals we receive from pulsars?
Where do the light waves come from?
To answer this question, we need to understand a complicated process that gives rise to the
signal; an exponentially growing shower of light and matter, all spawning from a single
electron.
Near the magnetic poles of a neutron star, an electron can be accelerated by the magnetic
field and emit a so-called curvature photon, tangential to the magnetic field line.
This marks, if you like, the start of a pulse.
The curvature photon moves in a straight line, until the angle between its momentum and the
magnetic field line becomes too great.
Once this angle reaches a threshold, the light dissipates and imbues its energy into the
quantum field of electrons.
An electron is created alongside its anti-particle, the positron.
The electron-positron pair has some momentum perpendicular to the magnetic field lines,
which they spontaneously dispose of in the form of synchrotron photons.
The two synchrotron photons can each then produce another electron-positron pair after
they reach the threshold angle between their momentum and the magnetic field lines.
This process repeats again and again, exponentially increasing the amount of photons and electron-positron
pairs created, until the synchrotron photons no longer have enough energy to create electron-positron
pairs, putting an end to the cascade.
These photons then beam out into space, while the original electron continues its journey
generating more curvature photons and more cascades as it moves along the magnetic field
lines.
This pair production cascade is why the light of a pulsar is so intense that we can detect
a signal from this tiny stellar remnant, thousands of light years away.
But why do we see this light as pulses, rather than a continuously glowing beacon of light
shining at us?
This is because the magnetic poles of a neutron star are rarely ever aligned with its axis
of rotation.
Just like on Earth, the magnetic north pole that our compasses point to isn’t the actual
geographic north pole that the Earth rotates around.
So the pulsars are like great lighthouses, sweeping their beams of light around the cosmos
as they spin.
And for an observer far away, like us on planet Earth, we see the beams sweep past us, again
and again, as the pulsar completes rotations on its axis.
These are the pulses of light that our telescopes can detect.
However, this is light with very long wavelengths in the radio part of the spectrum, meaning
only a radio telescope can detect it.
The light that the cascade produced is of the same wavelength, with the peaks and troughs
of the light waves fluctuating in unison.
The light waves are also polarised, which means they are all aligned along the same
axis.
This is a hallmark of neutron stars, and this is precisely what we observe in our 22-minute
signal.
The signal also has fluctuations that last between 0.2 to 4 seconds where the axis of
the light’s polarisation suddenly changes by 90 degrees, perpendicular to the original
axis, and then back again.
This effect is yet another signature of the cascades at the poles of the pulsars.
So much of our data points to a pulsar being the signal that you'd think this was an open-and-shut
case.
But one variable that we’ve mentioned earlier throws this entire theory into doubt: the
slow rotation rate of the neutron star.
Or rather, the combination of the slow rotation rate and the high stability of the rotation
rate of the source.
You see, as pulsars lose energy by shining their powerful beams into the cosmos, conservation
of energy will ensure that the pulsar slows down.
Eventually, the pulsar will slow down so much that it can no longer power the pair production
cascades, and the light emission starts to shut off.
The pulsar has entered the so-called ‘death valley’.
This graph plots neutron stars based on their rotation period on the x-axis, and the rate
of change of their rotation period on the y-axis.
Death valley is shown in this grey band running through the middle, and any pulsar that has
properties below this line should not be shining as the bright lighthouses they usually are.
We see that our signal is below even the lowest line marking the death valley, meaning that,
if it were a pulsar, it should be well and truly switched off.
And yet we are detecting it.
Look at the cluster of other known neutron stars on this graph.
They usually spin between ten times a second to once every second.
In comparison, our signal has a spin rate of once every 1318 seconds, over a thousand
times slower than the typical pulsar.
This would be fine if it was also slowing down quickly, which equates to moving this
data point upwards on this graph, above the death valley.
Such rapid energy loss would power the pair production cascade necessary to light the
beacon of the neutron star.
Yet the neutron star is mind-bogglingly stable, and it makes no sense that we can detect it.
The astronomers who found the signal considered an alternative mechanism that might explain
how a neutron star with such properties might have produced this light.
Maybe the neutron star is a magnetar – a neutron star that has an unusually strong
magnetic field, greater than ten thousand times the strength of the weakest neutron
star magnetic fields.
Magnetars are known to undergo ‘starquakes’ - cataclysmic events that release the tension
in the upper crust of a neutron star.
These stresses are produced by the strong magnetic fields of the magnetar, as well as
the slowing down of the magnetar rotation.
A fast spinning magnetar will bulge in the middle due to the centrifugal force distorting
the star from a perfect sphere.
As the magnetar slows down, the outer layers need to readjust to a new equilibrium and
lose some of the bulge they have.
The crust snaps to a new position, causing magnetic fields to temporarily realign and
powering the release of the energy as light that we can detect on Earth.
The most powerful starquake detected, that of SGR 1806-20 in 2004, released so much energy
that if it had taken place as far away as 10 light years from Earth, it would’ve caused
a mass extinction event.
If something is able to light the beacon of a dead pulsar, it would be this.
So, could GPM J1839–10 be a magnetar that has undergone a starquake?
Have we resolved the mystery of the 22-minute signal?
It seems not.
We expect these starquakes to also emit light in the x-ray part of the spectrum, yet no
x-rays can be detected from the position of the source roughly 18 thousand light years
away.
It also wouldn’t make sense for a magnetar outburst to be going on for over 3 decades.
The starquake is a temporary phenomenon, and the energy dissipates within a few years at
most.
It is simply incomprehensible that this signal would have existed for 35 years if it was
indeed a magnetar.
Once again, the unique properties of our signal exclude it from being a neutron star – even
an unusually powerful one that has undergone a special event such as a starquake.
But, what else could the source of this mysterious signal possibly be?
The astronomers who discovered the signal proposed a few alternatives for the identity
of the source.
One possibility is a highly magnetic white dwarf: A white dwarf is another type of remnant
left from the recent death of a star, but one that didn’t have enough mass to collapse
the empty space in the atoms to become a neutron star.
A remnant that has an unusually strong magnetic field could produce radio emissions, and as
it is not a neutron star, it could get away with being as slow and stably rotating as
the source of GPM J1839–10 while doing so.
The issue is, this would require an exceptionally strong magnetic field, greater than any we
have spotted on a white dwarf.
AR Sco is the only known radio pulsar that is actually a white dwarf, and its radio emissions
are around a thousand times less luminous than the source of our 22-minute signal.
So if a white dwarf is unlikely, what are our other options?
Astronomers have observed low-frequency radio waves coming from the interactions between
stars and exoplanets, as well as a binary of two brown dwarfs rotating around one another.
But this emission is typically even weaker, around 100 million times weaker than the source
of our signal.
In the end, it seems like none of our theories can explain the 22-minute signal.
While the unresolved question about the source of the signal may feel frustrating, this is
precisely the kind of mystery that astrophysicists look for.
When scientists find new data that challenges our long-held theories, they can usher in
revolutions in our understanding of the universe around us.
Here, our already shaky understanding of neutron stars is being challenged.
The astronomers are confident that the ease with which they identified this signal means
similar sources lie out there in the galactic plane, waiting to be identified.
Just like GPM J1839–10, the other signals might already be lurking in the data we have
collected.
Identifying more of these signals will shed light on the process powering emission beyond
the neutron star death valley.
Whatever lies behind the 22-minute signal, we are sure to learn of an entirely new phenomenon
that we have never seen before.
Isn’t that exciting?
What do you think could be the source of the 22-minute signal?
Let us know in the comments below!