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The Earth’s atmosphere protects us from a Universe that’s trying to kill us, but it also blocks our view of the extreme cosmos, like seeing x-rays and gamma radiation. Space telescopes are changing our view of the most extreme events in the Universe.
Show Notes
Introduction to the Electromagnetic Spectrum (NASA)
Electromagnetic Radiation (Swinburne University)
Alpha Beta Gamma rays (CNRS)
Gamma Rays (NASA)
X-rays (Swinburne University)
Fermi Gamma-ray Space Telescope (NASA)
What is a Cherenkov Detector (Radiation Dosimetry)
Cosmic rays: particles from outer space (CERN)
Tibet Observatory Confirms Existence of Galactic PeVatrons (Sky & Telescope)
PDF: First Detection of Photons with Energy Beyond 100 TeV from an Astrophysical Source (ArXiv)
Interstellar Gas Cloud (Swinburne University)
Neutrino, Cosmic Ray Discovery Puts Blazars in the Spotlight (University of Wisconsin)
How an accelerator works (CERN)
Transcript
Transcriptions provided by GMR Transcription Services
Fraser: Astronomy Cast, Episode 602. New colors or gamma radiation. Welcome to Astronomy Cast, your weekly facts-based journey though the cosmos, where we help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today and with me, as always, Dr. Pamela Gay. A Senior Scientist for the Planetary Science Institute and the Director of Cosmo Quest. Hey, Pamela. How’re you doing?
Dr. Gay: I’m doing well, how’re you doing, Fraser?
Fraser: Good. Oh, man. Summer has already arrived. We skipped spring; we’ve gone straight to summer. Temperatures are ludicrous here.
Dr. Gay: Oh, geez.
Fraser: It’s just, it’s amazing. Yeah. It’s so funny, it’s like one day, everything is dark and dismal and the next day, you’re wondering if this is the year, we buy an air conditioner. It’s crazy.
Dr. Gay: We’re having full on spring here today, but tomorrow it’s supposed to snow. So, I’m looking at our apple tree and hoping for the best right now.
Fraser: What?
Dr. Gay: Because it was in full bloom last week and whatever apples we’re gonna get have already been pollinated.
Fraser: Yep.
Dr. Gay: And I like apples.
Fraser: Yeah, yeah. Every year that some of my plants survive our winter, I’m always just shocked and surprised. You lived? Yeah. I was expecting it to not make it through that winter. We’ve had a couple. I’ve got a fig tree that’s not really happy with not being in the Mediterranean.
Dr. Gay: No.
Fraser: But apart from that. All right. So, the Earth’s atmosphere protects us from a universe that is definitely trying to kill us. But it also blocks our view of the entire cosmos, like seeing X-rays and gamma radiation. Space telescopes are changing our view of the most extreme events in the universe. And we will talk about this m ore in a second but first, let’s have a break. And we’re back. Okay, Pamela. I’m gonna be honest here, you put an episode on the schedule that I had absolutely no idea what it is that you were talking about. And not only that, I still have absolutely no idea what you’re talking about.
And I feel like normally, I am …I don’t know, a vacuum cleaner for every little tidbit of new space news and yet for some reason, you snuck one past the radar that I had no idea. So, I’ve decided I’m not even gonna prepare. I’m gonna discover this with the rest of the audience together while you tell us about this entirely new color that’s been added to the universe. Thanks, scientists. But before we get into the additional discovery, let’s start with what gamma radiation is.
Dr. Gay: So, gamma radiation is one of these terms that gets abused a great deal. Radiation is the problem. Radiation can mean two different things. It can mean either photons, just regular old colors of light traveling through space. Its cam mean radio waves. It can also though however get used to describe high energy particles. Alpha particles, beta particles. These are just nuclei of atoms in the case of alpha particles. They’re helium nuclei flying along at high velocities. And we call all of that stuff collectively, radiation.
Fraser: Right.
Dr. Gay: Now, gamma rays are just a color of photon, a wavelength, an energy that is higher energy than X-rays. So, when you shine X-rays through a human being, they get more or less stopped by your bones. They get less stopped by fleshy bits. And the ability of different parts of our anatomy to block X-rays in different amounts is incredibly useful for medicine.
Now, if you crank up the amount of X-rays, first of all they’re gonna go through you even more. And when they hit, they’re going to be more dangerous. And at a certain cut off point, we stop referring to things as X-rays, and this is entirely arbitrary. And we start calling it gamma rays. And everything at higher energies, no matter how much higher of an energy you get, everything at a higher energy. All of it. All of it is covered, is called gamma rays.
Fraser: Right.
Dr. Gay: So, we go from, in the colors our eye is dealing with, breaking it down to the ROYGBIV. Red, orange, yellow, green, violet indigo. Indigo, violet rather. To then having the UVABC based on well, getting sunburns and what our atmosphere stops. And then once we start getting into the uses for medicine, we have X-rays. And then everything else, gamma rays. Everything else.
Fraser: Right. And you neglected the other part of the spectrum as well.
Dr. Gay: We’re gonna get to that next week.
Fraser: Oh, okay. Okay.
Dr. Gay: So, next week we go redder.
Fraser: Okay. Okay. Right. And I think this was actually something that I didn’t learn until I was in my first year of university physics. I didn’t really wrap my head around it until I was actually taking university physics as part of my failed engineering career. That they’re all the same thing. That radio waves, infrared, visible light, X-rays, gamma rays, they’re all photons.
Dr. Gay: Yes.
Fraser: It’s just, they have different wavelengths along the electromagnetic spectrum. But you could turn one into the other. I mean, you can stretch out the wavelength of visible light and turn it into infrared. You can squeeze it together and turn it into ultraviolet. The color of the light that’s coming your way is just an indication of the wavelength. And in this case, as you say, the gamma radiation are the ones that are the smallest wavelength, the most squeezed together. And there’s infinitely smaller wavelengths that you can have. How do we detect gamma rays right now?
Dr. Gay: So, it depends on where you’re detecting them. So, if we go into outer space, we can directly detect them by using instruments, for instance Fermi is a gamma ray space telescope. And they don’t have the mirror and the lens set up like we have with optical telescopes or the dishes that we have with radio telescopes. Gamma rays really don’t like to be told what to do.
Fraser: Right.
Dr. Gay: And so, they have these complex internal scattering devices that basically funnel the gamma rays onto a detector that allows you to count photons in a particular direction in space. And here on the surface of the planet, we’re grateful to say, our atmosphere blocks gamma rays. Otherwise, we wouldn’t be getting a whole lot of spidermen, we’d be getting a whole lot of cancer, and that’s bad.
Fraser: Right.
Dr. Gay: So, what we have to do here on the surface of the planet, is we have detectors like Cherenkov detectors that look for the cascading particles that get generated when these high energy photons hit our upper atmosphere. And there’s starting to be a larger and larger variety of these on the surface of our world detectors that’re looking for essentially, the children of the interactions that occur when gamma rays get stopped by our atmosphere.
Fraser: Right. So, definitely need to go to space, or as you say, with the Cherenkov radiation, you can see the, I guess an indirect evidence of gamma radiation striking the atmosphere and causing a cascade of particles. Okay.
Dr. Gay: Yes.
Fraser: So, then what is the news?
Dr. Gay: So, there have been theories, guesses, hopes that we would be able to detect super-high energy gamma rays that originate from whatever is generating super-high energy cosmic rays. If you’ve gone out at night and tried to do CCD imaging for astrophotography, if you’ve turned your television on to a non-existent channel and it’s an old school television, you’ll see that static on the screen. That static is cosmic rays. Those bright splotches you sometimes get on your images during a CCD exposure, those are cosmic rays.
Fraser: Or when you’re an astronaut and you just close your eyes, those are cosmic rays.
Dr. Gay: Wasn’t gonna go there.
Fraser: Yeah. Yeah. Yeah. It’s one of the most unnerving things that astronauts will talk about. That they close their eyes, and they can see flashed of cosmic rays hitting their retina.
Dr. Gay: Yeah. Yeah. Wasn’t gonna go there.
Fraser: Yeah.
Dr. Gay: So, the problem with cosmic rays is, we can’t in general figure out exactly where they’re coming from because they’re charged particles. Some of them we can figure out because they’re coming from below us. Granite is a source of cosmic rays, by which I mean, nuclear decay is going on in the earth to generate particles as well. So, you can change the amount of cosmic rays that you deal with from the ground by where you put your telescope.
But as you go higher and higher up in the atmosphere, it was discovered you get more and more of these charged particles raining down on your detectors. And because they’re charged, as they travel through space, their paths constantly change by magnetic fields from stars, from our own atmosphere. From everywhere between here and where they originated. So, we detect these super-high energy particles, but because they’re charged, we have no idea where they came from.
Fraser: All right, well we’re gonna continue this story in a second. But first, let’s take a break. And we’re back. Okay. So, that is the challenge. What is the solution.
Dr. Gay: Yes. So, if charged particles, you can’t tell where they came from because the magnetic fields move their path, let’s look for things that don’t have charge. And this is where high energy photons, those gamma rays, come into being. Because you can end up with, they’re calling them PeVatrons.
Fraser: If it doesn’t have the name ray in it, I think they’ve missed the point, but sure. I guess microwaves, radio waves, gamma rays.
Dr. Gay: Green.
Fraser: Green.
Dr. Gay: Ultraviolet.
Fraser: Ultraviolet radiation. So, it would be PeVatron?
Dr. Gay: Yeah.
Fraser: PeVatron radiation.
Dr. Gay: And it’s for peta-electron bolt, is the amount of energy in these gamma rays.
Fraser: Right. Okay.
Dr. Gay: And they’ve been theorized for a long time. They’ve been gaining more and more popularity in the 2000s as we’ve gotten closer to having detectors capable of seeing these things. And the excitement is that if we see them coming directly from the source, it either was generated at that source or was bounced off of that source. And what we’re finding is two different families, it looks like, of these PeVatrons and a few really cool exceptions. So, the first low energy family that we’re finding is up on the Tibetan plateau, there is a high-altitude observatory that is looking for the child particles created when gamma rays in our atmosphere –
Fraser: Right, that indirect evidence I was mentioning earlier.
Dr. Gay: Exactly. And they’ve been able to trace back these reactions to the band of our own Milky Way. And what it appears is happening is all throughout the inside of our galaxy, we didn’t even know if these things were coming from inside or outside our galaxy, that’s how little we knew. What we’re finding is lower energy of these high energy gamma rays are essentially getting formed when cosmic rays hit the gas and dust in the plane in the Milky Way.
And then gamma rays are the result of this collision and they get sent flying our direction. We live in a high energy environment. We can’t see because of this atmosphere we have and all the magnetic fields around us and now we know our galaxy is a source of high-energy cosmic rays.
Fraser: But you’re saying that they’re coming from that way, right?
Dr. Gay: Yeah.
Fraser: They’re coming from the galaxy. But that doesn’t sound specific enough. There’s gotta be something that’s actually generating them. It can’t just be the galaxy.
Dr. Gay: And this is where it gets frustrating because, while we can say the cosmic rays hit the interstellar medium which is densest along the plane of the galaxy. And when it hits, generates gamma rays that we then detect. You still don’t quite know where those cosmic rays originated. And this is where it starts to become important, look at what instruments like IceCube is doing.
Where they’re starting to look for high energy particles. And then we have additional detectors looking at gamma rays and what we’re finding is, there’s one super nova remanent that gave off one of these peta-electron volt gamma rays. It appears Blazars are another potential source of these things. And this is where we have these additional populations that we see in the sky that are higher energy.
Fraser: Right.
Dr. Gay: And these higher energy sources are external to our galaxy and starting to hint at, in their numbers, in their directions. It takes an angry black hole or an exploding star, and by angry, I mean actively feeding on stuff. Black hole in the center of the galaxy. It takes that high energy environment to generate things that have gamma rays that are in a new name, peta-electron volt gamma rays.
Fraser: Okay, we’ll talk about that more in a second but time for another break. And we’re back. So, one of the big mysteries that astronomers have been puzzling with is just this idea, where are these cosmic rays coming with this amount of energy? And we’ve mentioned this in past shows, that they know each one comes with the energy of a baseball. One particle has a ludicrous amount of energy. Higher than anything we can produce in the large Hadron Collider. Higher than anything we could conceivably be able to produce. And so, we’ve talked about it before and our knee jerk reaction, black holes. Right? Super nova.
Dr. Gay: Yeah.
Fraser: There aren’t a lot of extreme events in the universe that you can go to, to try and explain this thing. So, then does finding these highest energy gamma radiation linked to these highest energy cosmic rays, is it fairly definitive now? To what we were suspecting before?
Dr. Gay: So, magnetic fields are certainly to blame. Within our own galaxy, we’re still struggling to figure out what all the sources are within our own galaxy, but it seems that super massive black hole in the heart of our own system generated cosmic rays once upon a time when it was a bit more active. Or during periods of activity and they’re still bouncing around generating X-rays for us. And then outside of our galaxy, Blazars, these are active galaxies that have massively feeding super-massive black holes in their center.
And the black hole itself, it’s wrong to think that it’s the magnetic field generating thing. As material tries to flow onto that black hole, angular momentum always wins, and material ends up forming a spiraling disc around that super-massive black hole. And any time you put charged material in motion, it generates a magnetic field.
Fraser: Yes.
Dr. Gay: More material, more charge, more magnetic field. Faster, more charge. Stronger magnetic field. Stronger, faster, denser magnetic field from stronger, denser, faster material. And Blazars are out there with these amazing jets of material that gets cascaded out of the magnetic field along its poles. And they’re amazing linear accelerators, essentially.
Fraser: And with a Blazar, are we staring down the jet?
Dr. Gay: It’s close to down the jet. One of the cool things about Blazars, is you can actually see what’s called superluminal motion. This is where you see something that appears to be moving faster than the speed of light, it is not actually moving faster than the speed of light. But the reason we see this perceived faster than light motion is, you have a jet of material with the galaxy in the center. And if we’re looking at it almost straight down the barrel, but not quite, we can see the ends of both jets.
And it takes the light from the further jet significantly more time to reach us. And so, when we measure how fast this appears to be moving and how fast the front jet appears to be moving towards us, if we don’t take into consideration the okay, this takes longer to reach us than that. It looks like the jets are moving faster than the speed of light. It’s a really cool just, geometry playing with our minds, kind of physics. But this almost, but not straight down the jet phenomenon allows us to see a lot more physics than we otherwise get to see.
Fraser: So, how do we use this? I mean, can we use this to identify potentially hidden galaxies that’re maybe obscured in dust? Can we use this to probe the environment around? Can we use this to measure the magnetic field strength of black holes? What’s it for?
Dr. Gay: So, at a certain level, it’s one of these things where our models for how powerful magnetic fields and some of these environments should be.
Say, we should be able to detect these things if we just build the right detector. These high energy particles should be out there. And so, you have it on the confirming theories side of things, and then on the other side of it, we know there’s these massively high energy cosmic rays that we can’t tell the origins of. And so, we can understand the cosmic rays, only if we understand and find, find first, then understand these gamma rays. So, they’re basically helping us understand cosmic rays, which is something that no one originally predicted. They just happened to be out there ruining our images.
Fraser: Yeah.
Dr. Gay: And now we’re trying to figure out how to understand them and where they come from and we’re confirming the theories that said, okay. So, we have cosmic rays, where do they come from? We think they come from here and if they do, we should also see this and this from neutral particles.
Fraser: Right.
Dr. Gay: On picture.
Fraser: And so, now we have an explanation for how the most extreme particle accelerators in the universe, where they are, how they operate and I’m looking forward to them. My favorite kinds of science results are the ones where somebody takes some extreme event like this where light echoes or something like that and goes, we can tell that this galaxy did a merger X billion years ago because we can see the light echo of the extra supernova that we’re going off during this time or something, right?
Dr. Gay: Yes.
Fraser: And so, I can just imagine them using somehow, the gamma radiation as it’s impacting the areas around the black hole to reveal information about the surroundings or previous generations of times when it was a quasar before or, who knows. Who knows? Now we at least know where they’re coming from and that’s pretty awesome.
Dr. Gay: And this is another side to that multi-messenger astronomy point. Where we don’t just observe the sky in light anymore. Light helped us figure out the cosmic ray particles.
Fraser: Yeah.
Dr. Gay: We now use gravity particles in light. It’s a single story unified, hopefully, by the occasional theorist.
Fraser: That’s awesome. All right, thanks Pamela.
Dr. Gay: Thank you.
Fraser: Now, do you have some names for us this week?
Dr. Gay: I do. As always, we are brought to you by you.
Fraser: By you.
Dr. Gay: You support Beth who is out there keeping our website up to date. Nancy, who runs herd over the two of us and we sure need Nancy. You’re supporting Allie, who puts together our videos. You’re supporting Rich who puts together our audio.
And this week, I would like to thank Gregory Singleton, Joshua Adams, Matt Newbold, Paul Disney, Chris Scherhaufer, Gfour184, Cooper, Dean McDaniel, Steven Shewalter, Father Prax, Scott Bieber, Anitasuarus, Rachel Fry, Dave Lackey, Andrew Stephenson, Anton Burgess, Lee Zealand, Cemanski, Planetar, Donald E Mundis, Jen Greenwalt, Bart Flaherty, Kenneth Ryan, Sean Freeman (Blixa the cat), Glenn McDavid, Kimberly Rieck, Benjamin Davies, Naila, Nial Bruce and Tim McMackin. Thank you so much, all of you for everything you do that lets us do what we do.
Fraser: Thanks, everybody and we’ll see you all next week.
Dr. Gay: Bye-bye, everyone.
Speaker: Astronomy Cast is a joint product of Universe Today and the Planetary Science Institute. Astronomy Cast is released under a creative common’s attribution license. So, love it, share it and remix it, but please, credit it to our hosts, Fraser Cain and Dr. Pamela Gay. You can get more information on today’s show topic on our website, Astronomycast.com.
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