LHAASO’s Discovery of a Super Cosmic Ray Source

Sep 13

Bare with me here, because I often use metaphors to explain physics concepts that are harder for me to grasp otherwise!

Now imagine walking through a giant and ancient forest.
It’s quiet at first, you’re used to the noise of cars in the city, but then you hear a bird call, then a bee buzzing by, and then the entire orchestra of all of the species that live there.
As you walk further into the forest of giant trees, the sound grows, then swells into something that almost reminds you of a busy city on a Saturday night.
That’s more or less what scientists have done recently, but in space: starting from faint little bleeps, then detecting a powerful ensemble of cosmic particles.
China’s LHAASO observatory has just made one of those cosmic explosions visible to us.

They’ve discovered what is being called a super PeVatron, which is basically a cosmic engine inside our own galaxy that slings tiny particles to mind-bending speeds, much much faster than anything we can create on Earth.
The proof of this engine is a huge glowing bubble of gamma rays in the Cygnus region of the sky.
It’s the first time we’ve clearly spotted a source of cosmic rays this powerful, reaching energies far beyond anything we’ve been able to trace before, which is kind of a big deal.

What did LHAASO actually find

LHAASO stands for Large High Altitude Air Shower Observatory, and it’s a massive cosmic ray/gamma-ray observatory high in the mountains of Sichuan, China.
It was built to see the showers of particles produced when cosmic rays or high-energy gamma rays hit Earth’s atmosphere. Which happens literally all of the time.

According to the internet of Google: “At sea level, every second, about one cosmic ray particle passes through your body. You don’t feel it, they’re that tiny and harmless at everyday doses. Across the whole Earth, about 10,000 cosmic ray particles hit every square meter per minute (that’s a shower constantly raining down).”

Now in February 2024, LHAASO announced that it had discovered a huge structure in the Cygnus star-forming region, a “γ-ray bubble” that glows with ultra-high-energy gamma rays.

Some specific facts about this bubble include: that the bubble spans at least 100 square degrees on the sky, it lives roughly ~5,000 light‐years away from Earth, and it is huge.

Inside this bubble, scientists have caught flashes of light (those gamma rays again) carrying mind-boggling amounts of energy, some a million billion times stronger than a single electronvolt (that’s 1 PeV)!
The most extreme spark they’ve seen so far is about 2.5 PeV, which makes our biggest particle machines on Earth look like toy car engines next to a rocket (it’s 1,000x bigger than our largest man made particle accelerator).

This evidence points to something completely wild hidden in the bubble: a “super PeVatron,” basically a natural space accelerator that can sling particles (like protons) to at least 10 PeV, maybe even twice that.
When those particles crash into the surrounding clouds of gas, they light up the sky in gamma rays, leaving the glowing fingerprints LHAASO has now spotted.

By studying the way the bubble shines (which parts glow brightest and how that matches the map of nearby gas) astronomers think the heart of this cosmic engine is Cygnus OB2, a gigantic cluster of hot, young, heavy stars.
And it may not be working alone; other massive stars and stellar remnants in the same neighborhood could be adding their strength to the mix.

Why this is a big deal

We’ve known for over a century that cosmic rays (high-energy charged particles) strike Earth continuously.
But there are still so many questions I have, including where do the highest energy ones come from?
What accelerates particles up past the “knee” in the cosmic ray spectrum? Also, what the hell is the “knee”?

The “knee” is a feature in the cosmic ray spectrum around a few PeV, where the intensity falls off more steeply.
Think of cosmic rays like raindrops falling on Earth.
If you count how many big drops versus little drops you get, there’s a pattern.
Scientists noticed that when cosmic rays reach a certain size (around a few PeV) the number of them suddenly drops off much faster, almost like hitting a step in the staircase.
They call that step the “knee.”
…different than the bodypart on my husband that is always hurting after he squats.

For decades, that silly little knee thing suggested a limit: most things in our galaxy just don’t have the power to sling particles much higher than that.
But LHAASO’s new discovery in Cygnus proves at least one source can!
This super PeVatron is blasting particles well past the knee, showing the Milky Way has accelerators far stronger than we thought.
Hitting below the knee is it?

When scientists mapped the glow of the bubble, they noticed something really interesting: the brightest spots lined up with clouds of gas floating nearby.
That’s a biiig clue, it means the gamma rays we see are probably created when super-charged particles slam into those gas clouds, lighting them up like the forth of July.

This points back to what must be hiding inside the bubble: a powerhouse that keeps flinging protons (and other particles) to ridiculous speeds before they crash into the gas.
And the best suspect is Cygnus OB2, that gathering of young, heavy stars.
If that’s true, it means giant star clusters like this aren’t just pretty stellar nurseries, they’re cosmic particle accelerators, playing a much bigger role in creating high-energy cosmic rays than we ever had proof of before.

What this means is that we’ve officially stepped into a new era of stargazing!

Our telescopes are now sharp enough to catch those tiny little light particles (photons) with energies in the PeV range, something we could only dream of just a decade ago.
That means we can do more than just say, “Yes, they exist.”
We can actually trace where they come from, sketch out the shapes of the regions that produce them, and study the way they shine across different energies!

It’s like suddenly getting night-vision goggles for the most extreme side of the universe.

The mystery

Of course, with every answer come a dozen new questions.
Here are some of the ones that pop up for me:

Could other objects (supernova remnants, microquasars) be contributing?

How does the process sustain continuous acceleration to tens of PeV? What astrophysical mechanism is in play (stellar winds, collective shocks, magnetic turbulence)?

It seems like the bubble contributes little to what we detect at Earth (for now). So while it’s crucial for understanding local cosmic accelerators, we need more sources to explain the full observed spectrum. Where are all the other rays coming from?

My thoughts

You know how in “Bees Can Do Math,” we discovered that bees aren’t just bumbling insects but are doing something surprisingly complex, quietly, and underappreciated?
This discovery is similar: the Milky Way isn’t just bombarded by cosmic rays from nebulous unknowns, there are big structures doing the work, turning gas, stars, and gravity into cosmic-ray factories.

I think of the Cygnus bubble almost like a “cosmic furnace bubble”: a shell around something hot inside, with gas filling it, particles bumping into that gas, producing gamma rays.
The really high-energy bits are like sparks flying off the furnace.
What’s elegant about all of this is that LHAASO doesn’t need to see the accelerator directly; it sees the glow, the fingerprints of what’s happening.

Sometimes we can’t see a power plant directly from our homes, but we see the lights it powers!
Or in a forest fire, you see the smoke even if you don’t see the fire right away.
Here, LHAASO sees the gamma-ray smoke; it gives away the fire of acceleration inside.

Also, this shows how sometimes the “big questions” in science (origin of cosmic rays) are solved not by small incremental tweaks but by leaps when you build new observatories sensitive enough to see something we couldn’t before.

Why this hardly appeared in Western popular science news

This is just my take, not from a press-release:

Many of the early reports came from Chinese institutions (IHEP, CAS) and in Xinhua, Global Times, China.org.cn.
Even though they publish in English, often these stories are less visible in Western media channels unless scientists in the West pick them up or write review articles.

Technical difficulty: The idea of ultra-high-energy gamma rays, “PeVatrons,” cosmic ray spectrum knees, etc., is niche.
It doesn’t easily translate into “alien spaceship!” headlines, which many media prefer (people click on those more)!
So a lot of these stories sit in astrophysics journals for a while unless someone like me gets bored and stumbles upon them.

Conservative scientific journalism also often waits until independent verification or wider commentary before blasting it all over the place.
Since LHAASO is newer, there are still details to be confirmed, models to be debated, and people to fight over what every little detail means.
Luckily, I’m here for the speculation and not to publish anything too serious or scientific, just spread news.

Also, a subtle bubble in the gamma rays doesn’t have the same crazy cool idea as a black hole image or a Mars sample return.
This is not super interesting unless it is found by nerds like myself.

What this discovery implies for the future

I’m guessing we’ll see more “super PeVatrons” identified, maybe even in the southern sky or in other star-forming areas!

Observatories will most likely improve with more sensitivity, better resolution, and even more energy range.
LHAASO itself may map more of the Milky Way and see similar bubbles.

Theoretical astrophysics will need to catch up with their models of cosmic ray acceleration soon too. Theories about how clusters of massive stars, stellar winds, supernovae, etc., combine to produce these acceleration areas in space will need to catch up.

If protons are accelerated to tens of PeV, interactions might produce not just gamma rays but potentially neutrinos that can be detected too, which would be awesome.

Let’s just keep in mind that this Cygnus bubble is probably not the only or the main source of cosmic rays at the highest energies…it probably isn’t.

We also don’t know every detail of the acceleration mechanism still.

And the contribution to what we detect on Earth is very small from this bubble, so it doesn’t solve all cosmic ray mysteries.

LHAASO discovery

The bubble LHAASO uncovered in Cygnus feels like one of those rare turning points, a moment when something hidden suddenly reveals itself.
What was once only theory now has a shape and a glow: a super PeVatron, stamped into the sky with energy so fierce it bends the limits we thought the galaxy had.
It’s not the whole story…but it’s a landmark, or a breadcrumb on the trail of cosmic rays that will lead us even further into the future.

I love that it reshapes how we picture the Milky Way.
Not just as a quiet scattering of stars and dust, but as a galaxy alive with engines, giant accelerators in the dark, flinging particles faster and higher than anything we can build on Earth.
Discoveries like this remind me that science doesn’t often hand us dramatic eureka moments, it’s more about patience and building new ways of seeing, tuning our ears to the faintest signs, until suddenly the universe reveals her secrets.

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Michele Gargiulo http://michelegargiulo.com