How Supernovas Act as Cosmic Particle Accelerators

[00:00:00]Hey Everyone. Before we get to the episode, just a heads up we have two new items at the  merch store. There’s a link in the description Cern's Large Hadron Collider routinely collides  particles at energies equivalent to a fraction of a second after the Big Bang. If this  worries you, then the following fact will either put you at ease or scare the hell out  of you. And that's that a particle with the energy of an LHC collision hits every square  kilometer of the Earth every single second.

[00:00:29]And we only relatively recently figured out  where these cosmic rays are coming from.

[00:00:37]—- In 1912, Victor Hess took a hot-air balloon ride  and got a Nobel prize. Hess had equipped the balloon with devices sensitive to electrically  charged particles. As the balloon rose, particle detections increased, quadrupling by the  maximum altitude of 5 kilometers. Hess wondering if these might be charged particles coming  from the Sun, and so repeated the experiment at night and during a solar eclipse. The  results were the same. He concluded that the vastness of space itself must be pelting  us with energetic charged particles. In this way cosmic rays were discovered. Hess’s  Nobel followed a couple of decades later.

[00:01:19]Now we routinely measure cosmic rays with a  wide range of advanced observatories—enormous arrays of ground detectors like the  Pierre Auger Observatory in Argentina, or orbiting detectors on satellites and the  ISS. But for a long time the mystery only intensified. We learned that the energies  of cosmic rays span 12 orders of magnitude, up to energies 10 million times that of the  Large Hadron Collider. The most energetic of all—the so-called oh-my-god particle—had  the kinetic energy of a well-thrown baseball.

[00:01:53]We did an episode on the oh-my-god particle  a while back, but didn’t get into the awesome details of how cosmic rays are really produced.  And we should probably cover that, because the origin of cosmic rays remained a mystery for  a century after they were discovered. It was only a decade or so ago that scientists came to a  general agreement about the origin of most cosmic rays—the ones without the most ridiculous energies  mostly come from exploding stars. From supernovae.

[00:02:24]We still don’t know about the more energetic ones,  but it turns out that by understanding the complex process behind the supernova-generated rays we  can come up with possible mechanisms for the rest.

[00:02:38]So let’s see how exploding stars can act  like particle accelerators. For that we need two things: shockwaves, and magnetic fields.

[00:02:48]Shockwaves are a big part of what makes an  explosion an explosion, versus say a bunch of hot gas expanding. They’re also a key ingredient in  making cosmic rays. We’ll start with sound waves before we get to shockwaves.

[00:03:01]These are pressure waves—oscillating patterns of  high and low pressure and density. We have some initial event that compresses the air, creating  a high density region. That region pushes away surrounding air molecules, creating a  new high density region down the path, which pushes on the following region, and  so on. The wave propagates at the speed of sound while individual air molecules  just move back and forth on the spot.

[00:03:24]A shockwave is what happens when the initial event  causes the particles to move faster than the speed of sound in whatever medium they’re in. That  means the particles will outpace any sound waves, and instead of bouncing backwards will continue  on, sweeping up the material they pass through.

[00:03:39]Familiar sources of shockwaves include supersonic  vehicles or meteors, which push the air ahead faster than the speed of sound. Or any explosion  in which the expanding material is supersonic.

[00:03:51]Shockwaves in space are pretty common, both from  explosions and from objects moving faster than the local speed of sound. Although it’s generally  true that in space no one can hear you scream, that doesn’t mean there isn’t sound in space. The  gas between the stars may be incredibly diffuse, but it’s still there, which means it can support  sound waves and so can also experience shockwaves.

[00:04:14]Perhaps the most famous type of space  explosion is the supernova. When the central core of a massive star has fused  itself into a giant ball of iron and nickel, it collapses into a neutron star and releases a  prodigious amount of energy in neutrinos. These sweep up the outer layers of the collapsing core,  sending them out a few percent the speed of light.

[00:04:33]This is much faster than the speed of sound in  the interstellar medium, so we have a shockwave.

[00:04:39]But a few percent lightspeed is not the  99.999 etc % light speed of some of the cosmic rays we’ve seen. It was Enrico  Fermi who figured out how supernova shockwaves can accelerate particles to  vastly higher speeds than they themselves traveled. This is the same Fermi who was  one of the founders of quantum mechanics, built the first nuclear reactor, and  wondered where all the aliens are.

[00:05:05]At around the same time that we were building  our first particle accelerators on Earth using powerful magnetic fields, Fermi realized  that magnetic fields might also do the trick in supernovae. We all know that the  Sun has a magnetic field. When that field gets tangled up, like it is right now, we  get magnetic storms. These frequently erupt, spewing out knots of magnetic field and  energetic plasma into the solar system.

[00:05:28]So if the Sun can do that over and over .. imagine what an exploding star can do.

[00:05:34]Let’s think about magnetism for a minute. Moving  electric charges make magnetic fields with field lines that loop around the direction of  motion. And magnetic fields generate a force on moving charged particles that cause  those particles to circle around the field lines. The net result is that magnetic  fields and the currents they generate are self-reinforcing. In a plasma, magnetic  fields sort of act like rebar in concrete, reinforcing the plasma, giving  it inertia, and also shaping it.

[00:06:03]So when a star explodes,  its magnetic field doesn’t just vanish. Some gets locked in the  resulting neutron star or black hole, but the rest is dragged outwards with  the rest of the explosion. The expanding shockwave is a tsunami of both charged  particles and of intense magnetic field.

[00:06:18]In an explosion in the atmosphere, the  shockfront high-density of air smashes into the lower-density air ahead of it, dragging it  into the propagating front. But the interstellar medium is so diffuse that we don’t expect a lot  of collisions with an oncoming supernova front.

[00:06:34]That would mean there is no shockwave if it weren’t for magnetic fields—but not just those in the shockfront. Weak tendrils of  magnetic field suffuse the entire galaxy, weaving through the interstellar medium. Our  supernova shockfront first grabs this ISM magnetic field and drags it forward, and this in turn drags the  ISM plasma. We call these collisionless shocks.

[00:07:02]Let’s think about the two components of the  shockfront. We have the matter. Material from the explosion is coming in fast. It slows down  in the shockfront because this is where we’re colliding with the interstellar medium. Meanwhile  material in the interstellar medium is being swept up and accelerated as it enters the front. So if  you’re a proton traveling with the shockfront, in your reference frame you actually see  material rushing in towards you from both sides.

[00:07:29]Next we have the magnetic fields. Behind you  there’s the magnetic field from the explosion, which piles up as it slows down approaching  the front, and ahead you have the weaker interstellar field which is also piling up.  These result in two walls of magnetic turbulence.

[00:07:45]So our proton is chilling in the shockfront  and bounces its way out, forwards or backwards, it doesn’t matter. There it finds itself in  a magnetic tangle that’s effectively moving towards it. It gets deflected back into  the front and is likely to pick up a bit of energy in the process due to the relative  motion of the deflecting field. And then it goes out the other side of the front and gets  turned around again, gaining a little more energy. This can happen many, many times,  accelerating the proton to enormous speeds.

[00:08:15]This is the Fermi Acceleration Mechanism, also  known as diffusive acceleration because the particles diffuse or bounce randomly through  the shock and tangled fields This is how we turn a supernova into a particle accelerator. In  fact the resemblance to an artificial particle accelerator is stronger than just the fact  that they both use magnetic fields. Why is the Large Hadron Collider so … what’s the word  … Large? It’s because larger loops can keep hold of faster charged particles. The turn radius  of a particle in a magnetic field is called the Larmor radius—or gyroradius or synchrotron  radius—same thing. This radius increases with the speed of the particle and decreases  with the strength of the magnetic field.

[00:09:01]Supernova remnants have magnetic fields not that  different to the Earth’s—really weak compared to a typical particle accelerator. But  they can still accelerate particles to a large fraction of the speed of light. The combination of  weak fields and high speeds requires an extremely large Larmor radius. And so for supernova  remnants to be good particles accelerators— without strong fields, they just need to be  ridiculously enormous. The turbulent regions can extend many times the width of our solar system,  while the overall radii of these expanding shells reach several light years. Plenty of space for  cosmic rays to bounce their way to extreme eneregies.

[00:09:43]Supernovae can energize a proton up to 10^17 electron volts. A supernova-accelerated  iron nucleus can reach about 10 times that energy. But the maximum energy of a  supernova-accelerated particle is still around 100 times lower than the highest energy  cosmic rays. So where do the rest come from?

[00:10:05]It’s not so easy to tell where any  given cosmic ray comes from because we usually don’t see them directly.  They tend to collide with molecules in the upper atmosphere and send showers  of particles towards the ground. But by collecting those particles we can get a  rough sense of the origin of a cosmic ray.

[00:10:22]The Pierre Auger Observatory is a giant cosmic  ray detector that spans 3000 square kilometers in western Argentina. It has found that  the origins of ultra high energy cosmic rays are scattered across the sky. If they  were from sources within the Milky Way then we’d expect a concentration in the plane of  the Milky Way’s disk. We can’t see exactly where these particles are coming from, however  their origin does appear to be correlated with the large-scale structure of the universe.  They’re likely coming from other galaxies.

[00:10:53]There are many potential origins for the highest  energy cosmic rays—perhaps in the extreme magnetic fields around magnetars or around the supermassive  black holes in active galactic nuclei. These cosmic particle accelerators look a bit more like  traditional synchrotron accelerators like the LHC.

[00:11:11]But there’s also a strong possibility that these  cosmic rays also come from scaled-up versions of the supernova shockwave. Shocks are observed  everywhere in the universe, and on all scales.

[00:11:22]Enormous shockwaves result from collisions between  galaxies, from the most extreme stellar explosions like hypernovae or gamma-ray bursts, and from  the jets produced by active galactic nuclei.

[00:11:34]This last one is especially promising. When  a feeding and magnetized supermassive black hole sends jets of material punching through its  surrounding galaxies, shockwaves are inevitable, and the Fermi mechanism will give us cosmic rays.  And when those jets punch into the intergalactic medium beyond the galaxy, they can bloom into  giant lobes of diffuse plasma and magnetic field.

[00:11:57]Although magnetic fields are weak in these lobes,  they span hundreds of thousands of light years, and so are able to accelerate cosmic  rays to the highest possible energies.

[00:12:06]It took a balloon ride and a century  of sciencing to be fairly sure that exploding stars are bombarding the earth with  cosmic rays. It’s going to take a bit longer to fully understand the most energetic of  these particles. Whether they’ll turn out to be from magnetars or supermassive black  holes or galaxy-scale magnetized shockwaves, the search is now on for the largest  particle accelerators in all of space time.

[00:12:32]How do you do fellow simulations? Before we end  the episode, I just wanted to let you know that we are adding a couple new permanent items  to the merch store. Back by popular demand is the return of the Be Quiet The Devs Will  Notice T-shirt, which now also available for the first time as a hoodie. And we also have  our new How Do You Do fellow simulations, T-shirt and sweatshirt. For those of you who'd  like to blend more seamlessly into the simulation.

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