From Earth, the night sky appears
ethereal, peaceful. It's so far removed
from human civilization. You'd be
forgiven for thinking we're immune to
anything going on up there. But that
couldn't be further from the truth. We
are under attack. Cosmic rays are
bombarding us from every direction. tiny
particles that collide with our planet's
atmosphere, setting off a chain reaction
of ionization that can render our
satellites and other electronic
machinery useless. But where do they
come from? For hundreds of years,
astrophysicists have searched in vain to
find the origin of these elusive
attackers with little success. Even the
type of source has evaded their
searches. But now, thanks to a whole new
field of research, we're starting to
find answers. Not only do we now know
what to look for, but they are proving
more powerful than we ever imagined.
What in the cosmos is possibly capable
of producing a quadrillion electron
volts of energy? I'm Alex Mccoan and
you're watching Astramm. Join me as we
follow the trail of cosmic rays, leading
us right to the limits of physics as we
know it. We'll see how scientists detect
the highest energy particles in the
universe and meet a new class of
astronomical objects whose extreme
behavior until recently seemed like the
stuff of science fiction.
The Milky Way is full of energy, but our
eyes can only detect a tiny fraction of
it. Beyond the spectrum of visible
light, charged particles can give off
higher energy radiation in the form of
X-rays and gamma rays, creating a mess
of energetic fingerprints throughout
space.
Astronomers can forensically decode
these cosmic clues to understand the
flow of energy through our galaxy. But
among these charged particles, there is
one group in particular that remains
clouded in mystery. Back in 1912, the
Austrian physicist Victor Hess made a
historic air balloon ascent up to 5,300
m where he could measure the rate of
ionization in the upper atmosphere, or
how quickly atoms and molecules are
becoming charged. He expected to find
that it decreased at higher altitudes,
confirming the prevailing theories at
the time. However, this was not to be.
Unexpectedly, Hess recorded a rate of
ionization that reached three times
higher than at sea level. This led to
the realization that the ionizing
radiation he had dedicated his career to
studying came not from Earth, but from
space. He had discovered cosmic rays and
they did not come in peace. Earth is
under constant barrage from them. These
high energy particles, mostly protons,
travel at nearly the speed of light and
collide with our planet's atmosphere,
sending a shower of secondary particles
down onto its surface. These attacks can
do some serious damage. The secondary
particles produced in cosmic ray showers
the likes of muons, neutrons, electrons,
posetrons and gamma rays can interact
with living organisms contributing to
genetic mutations and radiation damage.
And when cosmic rays interact with
satellites or other orbiting
electronics, they can trigger a bout of
ionization that can cause the circuits
to degrade or even fail
catastrophically.
And they are not only a nuisance to our
best space equipment. Cosmic rays have
meddled with our best scientists, too.
For more than 100 years, the question of
where cosmic rays come from has been one
of the major unsolved mysteries in high
energy astrophysics. Their physical
properties make them almost impossible
to track. Magnetic fields bend their
path before we can locate their origin,
and they break down into a shower of
particles before we can learn their
identity.
These challenges are laid bare when
scientists try to measure the cosmic ray
spectrum. If you plot the number of
incoming high energy particles as a
function of their energy, you notice a
distinct steepening point known as the
knee beyond which the really high energy
cosmic rays above four pa electron volts
are much less common. At first, some
scientists assumed that this knee must
mark the boundary between cosmic rays
coming from inside our Milky Way and
those coming from beyond. Others simply
didn't know. But just as scientists were
about to accept defeat, one observatory
stepped in to revolutionize the search
for these sneaky interlopers.
the mysteries of these incredibly high energy cosmic rays. The large high
alitude air shower observatory is a
groundbased observatory located nearly
4,500 meters above sea level in the
mountains of Sichuan province, China. It
has one main objective to find the
highest energy particles in the
universe. The chief scientist on this
mission is Professor Ten Cao who gave
Astramm an exclusive interview about
Lasso's work. He said to make good
measurements for the knee of the cosmic
ray spectrum, you need two things. The
detector must be big enough and it must
be able to identify the original
particle from the air shower it creates.
Cosmic rays become more rare the higher
the energy. For the highest energy
particles, less than one per square
kilometer per century is expected to hit
Earth.
To combat this issue, you need detectors
spanning a whole lot more than a square
kilometer, which is why lasso covers an
area the size of
190 football fields. As for identifying
the particles, I mentioned that the
cosmic rays produce a shower of
secondary particles when they collide
with the atmosphere. And this is what
scientists use. They must essentially
piece the debris together to work out
whether the original particle was a
proton, a helium nucleus, or something
heavier. This is a tricky process as you
need to make sure you catch all the
fallout. But Lasso is one of the most
welle equipped observatories to tackle
the challenge. It has at least three
types of interconnected detectors in an
array to capture the shower and then
identify the original cosmic ray
particle. This unparalleled ability
pretty quickly led to the team's first
big discovery. Professor Cow said, "At
the time, we only had around half of
Lasso built. We put it into operation
for around half a year and suddenly
found that there were so many gamma
rays. By 2021, Lasso had detected
gammaray photons with energies exceeding
one pa electron volt or one quadrillion
electron volts. and one at 1.4
quadrillion electron volts, making it
the highest energy photon ever observed.
For context, that's nearly 150 times
more energy than the fastest protons
accelerated by the Large Hadron
Collider. Now, the laws of energy
conservation tell us that PETA electron
volt particles don't just appear out of
nowhere. That colossal amount of energy
has to be transferred to the particles
from a source. Scientists had been
theorizing about such a source for
decades. But Lasso's finding proved its
existence once and for all. The
Pevatron.
Pevatrons are any source capable of
accelerating particles to a pael
electron vault of energy. And their
existence promised to revolutionize the
hunt for cosmic rays. But how do we find
them? Luckily, Lasso wasn't finished.
Gamma rays are useful because they are
neutral in terms of their charge, so
they can travel in straight lines
throughout the universe without being
bent by magnetic fields. This added a
new dimension to Lasso's work. It could
trace back to where the gamma rays had
come from. Mapping these traces across
the sky, scientists identified 12
distinct sources capable of producing
ultra high energy gamma rays, several of
which showed signs of accelerating
particles to a quadrillion electron
volts. There was a catch though. Lasso
may have identified the general
direction of these gammaray sources, but
scientists had no idea how far away they
were. 12 lines of sight stretching out
endlessly into space isn't exactly a
narrow search field. Except scientists
knew something else about these gamma
rays that would tighten the net. The
universe has been full of photons since
the big bang. And these photons are very
cold with low energies. Gamma rays at
ultra high energies are very likely to
collide with these background photons.
And when they do, they vanish.
This means that gamma rays simply can't
travel vast distances through space. So
if lasso was detecting them here on
Earth, then they couldn't have traveled
very far. In fact, there is no way that
these 12 high energy sources could be
extra galactic. They must be inside our
Milky Way.
Not only did this tell scientists that
the knee, the bend in the graph we saw
earlier, had nothing to do with cosmic
rays coming from outside the Milky Way,
but it brought the search for them much
closer to home. The race was on.
Now, before Lassa's work,
astrophysicists had developed theories
about the production of cosmic rays and
which type of source could be
responsible. After all, space is a weird
and wonderful place, home to several
unusual objects displaying extreme
behavior. But which of these oddities
were capable of producing the highest
energy particles? And how? For the past
70 years, a popular idea was that
supernova remnants played a key role in
producing cosmic rays. They certainly
have enough energy for it. In fact,
calculations show that if they converted
just 10 to 20% of their kinetic energy
into accelerated particles, supernova
remnants could supply most of the cosmic
rays we see in our galaxy.
See, when a star goes supernova, it
drives material out into interstellar
space, forming a collisionless shock
wave just ahead of it, where there is a
sharp change in density, magnetic
fields, and pressure. When a particle
hits this shock wave, it gets bounced
back and forth across the shock front,
gaining energy each time. Particles can
propagate here for some time up to
thousands of years. And the longer they
spend there, the higher the energy can
become. This process is known as
diffusive shock acceleration or the the
mechanism. But although supernova
remnants can accelerate particles in
this way, reaching better electron volt
energies may still be tricky.
Theoretical models suggest that
supernova remnants are only capable of
accelerating particles to these extremes
during the first 100 years of their
life. And all the supernova remnants we
know of are much older.
However, there might be one scenario
where supernova remnants can act as a
source for ultra high energy gamma rays.
If they gain enough energy to overpower
the magnetic forces that confine them,
some accelerated particles may escape
the supernova remnant and get trapped in
nearby giant molecular clouds. Here,
they can interact with the dense gas and
produce gamma rays with energies up to
one quadrillion electron volts. even if
the supernova remnant itself is no
longer capable of pael electron fault
acceleration.
So supernova remnants show some
potential as cosmic ray producers. Up to
six of the 12 lasso candidates could be
associated with them, but they're not
the main type of source we should be
looking at.
There's another source capable of
creating even higher energies, pulsars.
We've covered pulsars on the channel
before, so feel free to check out some
of the previous videos for more
information, but as a quick recap,
pulsars are highly magnetized rotating
neutron stars created in supernova
explosions. We have now detected over
1,500 pulsars. But how could they create
such high power gamma rays? Well,
they're already proven to be capable of
accelerating electrons and posetrons
since the dense and highly magnetized
structure rotates to produce powerful
electric fields. These electric fields
can strip electrons from the stars
surface forming a wind of particles that
expands outwards at rapid speeds. And
once this wind reaches the surrounding
material, like the gas or dust from a
nebula, it suddenly slows down, forming
a termination shock. At this shock
point, the particles are accelerated to
extremely high energies. You can think
of it like water flowing in a stream.
Normally, the flow is smooth, but if you
place an obstacle in the way, some water
will spill out, and this water will be
flowing much faster. The Lasso team
believed that more than 30% of the
candidate Everrons they detected could
be associated with pulsars, making them
a key player in the search for cosmic
rays.
The final type of cosmic ray source came
as a bit of a surprise to scientists. If
we take another look at the spectrum and
zoom into the knee shape, you see not a
smooth line, but a hump. In other words,
an anomaly.
The properties of these cosmic rays
didn't fit with them coming from a known
source like a supernova remnant or a
pulsar, which meant they must have been
created by another type of source. This
required scientists to think outside the
box and proposed some other less
traditional ways that particles could be
accelerated. And with that, they
recognized what could be described as
nature's ultimate particle accelerators,
black holes.
The idea that black holes could produce
high energy particles had been discussed
way back in 2009. Named the Bardos Silk
West effect after the scientists who
came up with it. It describes a
phenomenon where if two particles move
towards a black hole and collide near
the event horizon, they could reach near
unlimited energies. However, this theory
was thought to be effectively useless
since the particles would no doubt be
sucked into the black hole and lost
forever. That was until more recent
years when newer models revealed that a
fraction of the particles would more
likely be ejected back out into space.
Free of the black hole, these particles
could travel through space as none other
than cosmic rays.
Now, we've discussed some of the theory
behind cosmic rays and where they come
from, but it begs the question, what
have we actually found?
The main thing to understand about this
search is that it's really hard. Lasso
may have identified 12 candidate
Hevatrons in our galaxy, but finding the
actual object responsible is a whole
other task. That aside, the search is
still ongoing, and a few sources have
been found. So, without further ado,
let's take a look.
One of the first objects pinned down may
be familiar to you. The Crab Nebula,
known as Messia 1, it's a supernova
remnant found around 6,500 lighty years
away in the constellation Taurus.
Although stunning to look at, Messia 1
is not just a pretty face. It's capable
of accelerating electrons to a
quadrillion electron volts of energy.
And as one of the best studied objects
in the known universe, observing Messia
1 gives astronomers a good insight into
how nature's particle accelerators work.
In the gammaray domain, Messia 1 shows
some extreme variability. It produces
intense flares which can last anywhere
between a few hours to a few days. And
with our new understanding of pevetrons,
scientists realized that these flares
were the photons resulting from some
serious electron acceleration.
Exactly how this happens has been
debated. It could be DSA at the boundary
between the particle wind and the medium
surrounding the pulsar, energy released
by magnetic field lines breaking and
reconnecting, or a more complex
mechanism within the particle wind
itself. For context, electrons at high
energies transfer part of their energy
to background photons, boosting them to
gamma rays that scientists can detect.
However, accelerating electrons is
really difficult because they lose
energy very quickly. To produce gamma
rays with energies of a quadrillion
electron volts, the electrons themselves
must have had several times that energy.
This proves that Messia 1 is undoubtedly
a Pevotron and an impressive one at
that. However, this has only been proven
for electrons, making Messia 1 what
scientists call electronic accelerator.
But pevetrons are capable of
accelerating any charged particle. And
if you remember, cosmic rays are mostly
protons. So, it's these pevetrons,
otherwise known as hydronic
accelerators, that scientists are most
keen to find.
I mentioned that lasso detected a photon
at 1.4 peta electron volts, the highest
energy photon ever observed. With such
mindblowing energy, scientists were keen
to see where it came from, which led
them to our next candidate, the Signis
region. The Signis constellation is one
of the most recognizable in the northern
sky, spanning more than 800 square
degrees or 4,000 full moons. And it was
here that scientists found some
seriously interesting stuff. Lasso found
lots of petal electron vault photons
inside the Signis Cocoon, a huge superb
bubble which surrounds a region of
massive star formation.
Inside the bubble is a massive young
star cluster known as the Signis OB2
Association. This is a very active place
and the many young massive stars can
create a strong wind which accelerates
particles. Signis OB2 is found nearly
5,000 lightyears from Earth and it lines
up pretty well with some of the gamma
rays observed by Lasso. But as I
mentioned, these gamma rays only
indicate the general direction of a
pevetron, not its distance. So
scientists can often struggle to
identify the origin of a signal among
several possible sources. And in the
case of Signis, there are plenty of
energetic objects to choose from. In a
similar direction to OB2, but further
away, there is an X-ray binary known as
Signis X3.
It consists of a donor star and a
compact object likely a black hole or
neutron star. The donor is a wolf ray
star meaning it is extremely hot and
massive and it feeds material into the
compact object through strong stellar
winds. This combined with the compact
object which releases powerful jets of
plasma creates the perfect conditions
for particle acceleration.
At first, it seemed impossible to tell
which system Lasso was detecting PA
electron volt signals from. But on
taking a closer look at the signal
itself, scientists noticed something
unique. There was a temporal feature of
the signals. A pattern that repeated
every 4.8 hours, seen not just in the
gamma rays, but in the X-rays and the
infrared radiation, too. But where was
the pattern coming from? On further
investigation, scientists realize the
truth. Every 4.8 hours, the black hole
of Signis X3 orbits its massive donor
star. Suddenly, scientists could be
certain the gamma rays were coming from
Signis X3 and that they had found
another Pevotron.
Signis X3 was a particularly intriguing
object. For one thing, the highest
energy photons from this object already
measured 3.7 pa electron volts. And
since photons are radiated by
accelerated protons, the energy of the
proton must be several times larger.
What that suggests is that Signis X3 is
not just a pevatron, but a super
pevetron capable of accelerating protons
to at least 10 pa electron volts.
We've had a whistle stop tour of some of
the most exciting pevatrons we found so
far, but what does the future hold for
this ultra powered search? And where
will it take us? Hunting 12 candidate
pevetrons seemed hard enough, but since
Lasso's first finding, that number has
boomed. Lasso published the first global
catalog of galactic pevatrons in 2023
detailing 43 ultra high energy gammaray
sources. But now 2 years later and with
continued monitoring from observatories
around the world the number of sources
has risen to more than 75.
With so many candidate pevatrons hiding
in our Milky Way, we need to seriously
rethink our perception of the galaxy and
what it's capable of. Many thought that
the Milky Way was a relatively peaceful
place since it lacks the massive black
holes typical of galaxies with violent
energetic pasts. However, Lasso's work
has proven this to be an outdated
assumption and is changing the landscape
of our non-thermal universe, starting
with our own cosmic home. It's
definitely an exciting time to be
hunting cosmic rays. Lasso is
collaborating with several other
observatories detecting high energy
gamma rays around the world, including
the US, Germany, Namibia, and Spain.
This international network has fostered
an open and collaborative approach to
their experiments, sharing data and
cooperating evidence. Their goal is to
build up a comprehensive spectrum of
photons across different energies as
well as measuring X-rays and gamma rays
above Earth's atmosphere.
I want to give a huge thanks to
Professor K for his expertise on the
world of high energy astrophysics. As
I'm sure you'll agree, studying
peveratrons is a dynamic and rapidly
developing field, and observatories like
Lasso are on the front line. Their work
so far has transformed our galaxy from a
tranquil place to a violent, energetic
mess filled with particle accelerators,
more powerful than anything we can build
here on Earth. We may still be under
attack from cosmic rays, but now we're
ready to chase them back to their hiding
places.
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