﻿Eight hundred light-years away,
there’s an unseen anti-matter factory churning out high-energy positrons.
Tiny particles of antimatter that are streaming through the cosmos and colliding with our planet.
For much of history we didn’t know this strange source existed,
most of the positrons bombarding us went completely undetected,
instead getting absorbed in our planet's atmosphere.
It wasn’t until we started looking from beyond the bounds of our planet that we noticed them.
In 2011, NASA’s Alpha Magnetic Spectrometer -
a state-of-the-art particle detector some 200 miles up aboard the International Space Station -
was switched on. What did it find? You guessed it, positrons.
The presence of these subatomic particles was to be expected,
but not in the numbers they were finding.
Such was the sheer volume of positrons being detected,
that the usual sources like natural radioactive decay
and cosmic rays no longer offered a sufficient explanation.
So, where were they coming from?
We’ve only recently been able to trace the culprit of this cosmic anti-matter shower,
and it all comes down to another high energy discovery:
A strange gamma-ray haze named Geminga, first identified in the 1970s.
What is this mysterious source of gamma radiation?
And what does it have to do with the unusual abundance
of high-energy positrons hitting our planet?
whose gamma radiation has lit up the world of astrophysics for decades.
Up in our night sky,
nestled in the Gemini constellation in the Northern Celestial Hemisphere,
there is something peculiar going on.
In 1972, NASA’s Small Astronomy Satellite 2, or SAS-2,
identified an unknown source of gamma radiation. But with the technology available at the time,
the best it could do was trace its origin to this wider region of our Milky Way.
So the radiation’s ultimate source remained hidden among the stars for decades. Nevertheless,
it was given a name … Geminga, as coined in 1976 by Italian physicist Giovanni Bignami,
who would dedicate his career to studying it. It’s a play on words – a combination of “Gemini”,
the region where it’s located, and “gamma”, the type of radiation it
emits. Geminga is also a pun in Bignami’s Milanese dialect meaning “it is not there”;
a fitting name for a gamma-ray haze with an unknown origin.
It wasn’t until 1983 when Bignami and his team finally had their big break. They managed to
identify a weak X-ray signal from Geminga using the Einstein X-ray satellite. This meant although
its exact position remained unknown, they could narrow down their search area and were getting
closer to uncovering Geminga’s hiding place. But it didn’t answer the big question - what
is it? - as astronomers could still only offer vague guesses about the true nature of the source.
That was until 1991, when they had another lucky break.
Two separate missions identified radiation coming from Geminga; and they weren’t constant signals,
but pulses. The first of these discoveries was made with a German-built X-ray telescope,
known as ROSAT – short for Röntgensatellit, named after the German word for X-rays.
ROSAT was the first to identify pulses in the X-Ray signal coming from Geminga, and soon after,
they were also confirmed in the gamma wavelengths, by the Energetic Gamma Ray Experiment Telescope,
or EGRET, a telescope aboard NASA's Compton Gamma Ray Observatory satellite.
Not only did these complementary observations demonstrate that the X-rays and the gamma rays
were both coming from Geminga, but for the first time, they revealed
what Geminga was. With a period of 0.237 seconds, flashing as it spins around its
axis a little more than 4 Hz, or 4 times per second, Geminga behaved like a pulsar.
A pulsar is a type of neutron star that spins rapidly, emitting beams of radiation that sweep
across space like a cosmic lighthouse. From across the galaxy, most pulsars appear to
flash in radio waves, anywhere from a few times a minute, to as fast as 700 times per second.
And at this point in the early 90s, they were incredibly rare. You see,
before ROSAT and EGRET only two other high-energy gamma-ray pulsars had ever been identified:
the Crab and Vela pulsars. And Crab and Vela were different to Geminga in a couple of key ways.
First, in addition to gamma-rays, both of these pulsars also produced radio waves
and were therefore visible using radio telescopes. So if Geminga was a pulsar,
then it would be the first discovery of one that was apparently radio silent,
only emitting enough radiation to be seen in the gamma and x-ray wavelengths.
And second, Crab and Vela are surrounded by their respective nebulae – remnants from when
they were created from supernova explosions. But Geminga’s nebula was conspicuous by its absence.
So why is Geminga – this powerful source of gamma-rays – so good at hiding from our radio
telescopes? Where is its nebula? Or could it be a different type of object altogether?
Well, the answer to the first question is, in part, because we hadn’t been listening properly,
due to the limitations of the available technology and due to our understanding
of the radio emissions of such stellar remnants.
You see, while radio pulsars can emit radio waves across a wide bandwidth from as low as
17 megahertz, to above 87 gigahertz – around half of the radio spectrum – not all of these
frequencies travel well through space. Even though we’ve known since the 1970s
that radio pulsars often peak between 100 to 200 megahertz, where they are intrinsically brightest,
things like the interstellar medium, background sky temperature, and effects from the ionosphere,
mean that lower frequencies are dampened as they make their way across space,
resulting in very weak signals that are more difficult to detect.
Because those radio signals are so weak, most radio telescopes hadn’t been looking for them,
instead confining themselves to searching for signals between 430 and 1600 megahertz.
This would have been fine had Geminga behaved as expected for one of its kind.
Since it did not, it took until 1997 for scientists to realise what was happening.
Three independent observations from the Pushchino Radio Astronomy
Observatory were able to identify extremely weak pulses from Geminga,
using a sensitive transit antenna. The faint radio pulses came in around 100 megahertz,
which explains why previous radio searches for Geminga had come up silent. Turns out,
Geminga wasn’t truly hiding and had been sending us signals; we just weren’t listening correctly.
That same year a team led by the late astronomer Janusz Gil theorised that another reason Geminga
had appeared to be radio-silent may be its magnetic field. Models showed that the radio
waves may be absorbed or refracted within the pulsar’s magnetosphere,
leaving only weak pulses around 100 megahertz to be detectable. This would
effectively leave it quiet at the higher radio bands that most telescopes used.
Confirming Geminga was a pulsar - and specifically a gamma ray pulsar - was a big deal. In fact,
99% of its output is in the gamma range, making it one of the brightest gamma-ray
sources in our entire galaxy. It is, as it turns out, all that’s left after a star,
several times more massive than our Sun, exploded about 350,000 years ago.
But its relative radio silence and its apparent
lack of a nebula weren’t the only unusual things about this pulsar.
When Italian astrophysicists including Bignami, Geminga’s name-giver, compared a series of
observations from the European Southern Observatory’s ESO 3.6 metre telescope and
New Technology Telescope with observations from the Canada-France-Hawaii telescope,
they found that Geminga was moving. Not only that,
it was travelling at an unusually high speed of around 0.2 arcseconds per year.
As a reminder, an arcsecond is a very small unit of angular measurement, used when we need more
precise measurements than a degree would allow. Within each arc degree there are 60 arcminutes,
and within each arcminute are 60 arcseconds. These arcseconds are a common unit used in
astronomy to talk about the movement of objects across the sky, from our perspective on Earth.
If you were to draw a circle around the orbit of the moon around the Earth,
there would be 360 degrees around that circular path. So at any given time of day or night,
assuming nothing is blocking your view of the horizon, you can see about 180 degrees of the sky,
and from the horizon to the zenith, the top of the sky is 90 degrees. If you hold
out your little finger at arm’s length and close one eye, the tip of your little finger
covers about one degree of the sky, roughly. Next time you’re outside on a clear night,
try this, and see if your little finger can cover the moon – it should,
because the moon takes up only about half of a degree, or about 31 arcminutes in the night sky.
Geminga travelling 0.2 arcseconds across our sky each year may not sound like a lot, but from our
perspective on Earth, the typical star only moves a few thousandths of an arcsecond per year. Yet,
despite being 800 light-years from us, Geminga will travel 30 arcminutes – the
equivalent to the apparent diameter of the Moon – across our sky in just over 10,000 years.
In other words, this stellar corpse is racing through the
galaxy at nearly 210 kilometres per second.
Heading toward the border between the constellations Gemini and Lynx,
and at its current rate of motion, Geminga will remain in Gemini for
another half million years – but it may need a new name after that.
However, this mysterious pulsar gets stranger still. It’s its vast speed that helps produce
another feature that scientists were about to discover. As it hurtles through space,
Geminga leaves behind two ghostly X-ray tails that streak 3 trillion kilometres across the sky.
As I discussed earlier, despite the fact that this fast-moving pulsar is nearly radio-silent,
it certainly isn’t quiet in the gamma-ray and X-ray wavelengths.
In 1999, ESA’s X-ray Multi-Mirror Mission, or XMM‑Newton, was launched to peer deeper into
this X‑ray universe, and four years later, a team led by Patrizia Caraveo uncovered these
comet-like X-ray trails. Their shape and brightness are partly explained
by the shockwave created by Geminga’s motion through space, and its rotation
as a pulsar. But they are also revealing of another attribute… Geminga’s colossal mass.
Measuring only about 20 to 30 kilometres across, Geminga is extremely dense,
containing about as much mass as one and a half of our Suns. To put that in perspective,
if you had a teaspoon of neutron star material,
it would weigh about 4 billion tonnes – as much as 10,000 Empire State Buildings.
As this dense, high mass object races forward through the low-density
interstellar medium – just 0.06 to 0.15 atoms per cubic centimetre – it
compresses the interstellar medium and its own embedded magnetic field by a factor of four.
Meanwhile, the incessant spinning of the neutron star creates an environment where electrons,
and their anti-matter counterparts called positrons can be accelerated to extreme
energies, powerful enough to emit high-energy gamma-rays.
While most of these electrons are seen in the gamma radiation that escapes from the pulsar,
some get trapped and spiral within this enhanced magnetic field. In these images from a computer
model, the tails can be seen streaking along the edges of Geminga’s three-dimensional shockwave,
like the wake created by a boat going through water – only this boat is more
massive than our Sun, and the wake is made up of extremely high-energy X-rays.
But the final piece of the Geminga puzzle wasn’t discovered until 2005… its nebula.
Taking the form of a shell of neutral hydrogen gas with a radius of 0.4 parsecs wide,
it turned out to be what we call a “pulsar-wind nebula”.
This type of nebula is created from the wind of plasma that emanates from a pulsar's magnetic
poles. The plasma, made of charged particles that can be accelerated to near light speed, surrounds
the pulsar, creating a nebula of high-energy particles that give off strong X-ray emissions.
With the confirmation that Geminga did have a nebula,
its identity as a pulsar could finally be confirmed. But Chandra went even further.
In addition to imaging Geminga, Chandra also looked at a second pulsar, called B0355+54,
and by comparing the two, astronomers uncovered
another possible explanation for the absence of radio pulses from Geminga.
On the surface, these pulsars seem quite similar. They are both about
half a million years old, and they spin about 4 or 5 times per second.
However, as you know, Geminga is seen primarily in gamma-ray pulses,
with no bright radio emissions. By contrast, the other pulsar, which I’ll refer to as pulsar B,
is not seen in gamma rays, and instead is one of the brightest known radio pulsars.
How could these two pulsars be so similar, yet so vastly different in how we see them?
The answer may be as simple as how each of these pulsars are oriented,
relative to our observation from Earth.
Astronomers believe that these images of Geminga, and pulsar B,
have revealed their spin axes, and uncovered a reason for why radio and
gamma-ray pulses may be present, or absent, on different pulsars.
Like our own magnetic field around Earth, both of these pulsars have magnetic poles
close to their spin poles. These poles are where the beams of pulsing radio emissions
come from. You could try to model this if you skewer a little foam ball right
down the middle. The foam ball is a pulsar, and the radio beams coming
from the poles are represented by the wooden skewer coming out both ends. If you spin the
ball around the skewer like a spin axis, you create an equator around the middle.
To illustrate the gamma-ray source along the spin equator, called the torus,
you could cut a hole in a paper plate and squeeze it over the foam ball - now you’ve
got yourself a disk of gamma-rays beaming out from the equator in every direction.
With Geminga, the edge of the paper plate is pointing towards us,
meaning the gamma-rays are heading to Earth. But for pulsar B, its relative
position to us is at a different angle, as if looking at the flat surface of the
plate. The gamma rays are moving perpendicular to our line of sight, therefore missing Earth.
Let’s look again at the two Chandra images of Geminga, on the left, and pulsar B, on the right,
along with artist illustrations of what astronomers believe the pulsar wind nebulas look
like for each of these … In the image of pulsar B, the long trailing blue tails represent the radio
jets emanating from its poles, and the skewer coming out of both ends – only instead of being
straight like a wooden skewer, pulsar B is moving so fast through space that these jets appear bent
backward, trailing behind as the pulsar moves through space. Now look at the image of Geminga
– here, the long twin tails on either side of the image are the radio jets, trailing behind as it,
too, rushes through space. But this time instead of pointing almost directly toward and away from
our vantage point on Earth, these jets appear to be pointing off to the sides, not aimed at Earth.
So when astronomers look at Geminga, they see powerful gamma-ray emissions from the spin
equator, but the radio jets point to the sides and remain unseen. And when they look at pulsar B,
the opposite happens – the radio jets are pointed almost straight toward our planet,
while the gamma-ray source at the equator is missing Earth.
Sometimes, the most simple explanation is the correct one.
And that finally brings us back to the decades-long
mystery of an unusual abundance of anti-matter bombarding our planet.
For more than a decade, a particle detector called the Alpha Magnetic Spectrometer,
or AMS-02, has been attached to the International Space Station,
collecting information on anti-matter, dark matter, and cosmic ray sources. As a reminder,
cosmic rays are energetic particles – fragments of atoms – that travel through space at nearly
the speed of light. These can be made by the Sun, by supernova explosions, or other cosmic means.
And in 2013, the first results of the AMS-02 experiment were announced – the detector had
recorded more than 400,000 positrons, the largest sample of cosmic ray positron data ever collected,
and increasing the world’s total cosmic ray positron data by a hundredfold.
For years, many astronomers and physicists hoped that this excess anti-matter may be
the by-product of dark matter annihilation, offering possible clues about this mysterious
substance. After all, dark matter could make up around 27% of the cosmos, and yet,
we still don’t know what it is. Like regular matter, dark matter holds mass and takes up space,
but it doesn’t seem to absorb, reflect, or interact with light, at least not in
a way we can detect. Some theorise that dark matter may be made of yet unidentified types
of particles. Whatever it is, scientists had high hopes that the over-abundance of
anti-matter being detected aboard the ISS may hold clues about dark matter’s true nature.
Unfortunately they’ve been left disappointed. The more scientists dig into the data,
the clearer it’s becoming. The most likely source of these positrons may actually be
pulsars. Astrophysicists had long suspected this, but until 2017, there simply wasn’t proof.
It was the High-Altitude Water Cherenkov Gamma-ray Observatory (HAWC) that finally
added evidence to this hypothesis. A small halo of gamma radiation was
identified surrounding Geminga with trillions of times more energy than is visible to our eyes,
from 5 to 40 trillion electron volts – the sort of radiation usually produced by positrons. This
was the first real observational evidence pointing to a pulsar as a potential source.
Pulsars naturally surround themselves with a haze of both electrons and their positron counterparts,
as a result of the star’s intense magnetic field. This intense magnetic field pulls particles from
the pulsar’s surface and accelerates them to near the speed of light. Scientists
think that these accelerated positrons and electrons are then colliding with starlight,
boosting the light to higher energies, which then radiates as the gamma-ray halo observed.
But, based on the size of the halo the HAWC team saw, Geminga’s positrons would
rarely have the energy required to reach our planet, and so they believed that the excess
positrons must have a more exotic source. That was, until stunning new information
was uncovered a few years later thanks to a team led by astrophysicist Mattia Di Mauro.
Using a decade of gamma-ray data about Geminga, acquired from Fermi’s Large Area Telescope – which
is able to observe lower-energy light than the HAWC Gamma-ray Observatory
– Di Mauro’s team was able to subtract out all other gamma-ray sources to reveal a spectacular
glow coming from Geminga, much, much bigger than what scientists had ever seen before.
This vast, oblong halo of glowing gamma rays at an energy of 10 billion electron volts,
spanned 20 degrees of the sky – similar to the area the Big Dipper constellation
occupies. And that’s not all … the glow of gamma radiation is
even bigger at lower energies. If we could see it with the naked eye, Geminga’s gamma
glow would dominate our sky, covering an area 40 times bigger than the full Moon.
With this new information, astrophysicists found that the size of Geminga’s halo meant this one
pulsar alone could be responsible for as much as 20 percent of the excess positrons detected
near Earth. From there, it’s no stretch to imagine that other pulsars are the most
likely culprit for the remaining antimatter abundance we found. This explanation may not
have solved the mystery of dark matter, but it is certainly a magnificent revelation.
It was Jocelyn Bell Burnell who discovered the first pulsar in 1967,
back when people thought that those regular signals could be the work of
extra-terrestrial life. In the nearly sixty years that have passed since, we have found
thousands of pulsars, and our understanding of these neutron stars has grown with every one.
And since Geminga was identified as only the third known gamma-ray pulsar in 1991,
we’ve now spotted over 300, thanks to NASA’s Fermi mission. But given
Geminga’s track record of defying expectations and furthering science,
I like to think that this particular pulsar has more secrets still to reveal.
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