And now we reach the end of our tour through the electromagnetic spectrum. Last stop… gamma rays. These are the most energetic photons in the Universe, boosted up to incredible energies in the most violent places in the Universe. Gamma rays are tricky to catch, but they can reveal the most dramatic events in the Universe.
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Fraser Cane: Hello Pamela. We’re a little late because I have a dead computer.
Dr. Pamela Gay: We’re very sorry. We’re going to make it up for you. We’ll be throwing out lots of episodes in the next week or so.
Fraser: I know this isn’t very scientific but Macs seem fragile to me. I’ve gone through two motherboards and a hard drive. I’m really glad that I have a back up. I’m really glad that we have AppleCare.
Fraser: I’m sure I’ll get e-mails from a person that their Macs lasted them for six years and has been thrown off of cliffs and all that but mine is a delicate fragile flower. Thanks everybody who posted reviews of the astronomycast on iTunes. It helped inflate our egos immensely. We really appreciate that.
Now we’ve reached the end of our tour through the electromagnetic spectrum, last stop gamma rays. These are the most energetic photons in the universe boosted up to incredible energies in the most violent places in the universe. Gamma rays are tricky to catch but they can help reveal the most dramatic events in the universe.
Pamela let’s get our bearings again here. We’ve gone through radio, visible, ultraviolet, x-ray, infrared and here we are now at the end of the spectrum, gamma rays. Give us sort of some wavelengths.
Pamela: We talked about last week the x-ray spectrum ends at the short side around (point one) .1 nanometers. That’s where gamma rays pick up. These aren’t hard and fast divisions depending on who you’re talking to and what telescope they’re using.
There’s overlap in where we say x-rays and gamma rays begin. Anything shorter than that anything that has a smaller wavelength than that is said to be a gamma ray.
These are the little bits of light the photons that are capable of completely destroying pieces of DNA and cells. This is the nasty dangerous luckily for us stopped by our atmosphere color of light.
Fraser: I guess there’s no lower limit on how small the wavelength can be but physicists measure photons in terms of energy, right?
Pamela: Yes and this is where we start talking about stuff that has mega electron volts worth of energy.
Fraser: You’ll see that when you see someone mentioning gamma rays they will say like capital m little e big v, right?
Pamela: Right and the Fermi telescope that is used to detect these gamma rays – it used to be called GLAST now renamed Fermi is looking for light that has energies at anywhere from 30 megaelectron volts up to 300 gigaelectron volts. This is high energy would like to kill you light.
Pamela: Yeah, we’re all about death.
Fraser: Right so we’ve got kind of some parameters but there’s no theoretical I guess limit then to how small. If it is smaller than that .1 nanometer it is a gamma ray and it doesn’t matter if it is tinier and tinier it is still a gamma ray.
Fraser: Mostly astronomers measure it in terms of kilo and megaelectron volts because then they have a big number that is just going up and they can get kind of a fine nuance measurement to it.
Let’s talk then about detectors. What is the kind of instrument that is required to sense a gamma ray?
Pamela: There lots of ways to do this. You can actually start measuring these by what is the effect that they have when they hit something in terms of it is a scintillation detector.
You have for instance something like a sodium iodide crystal that when it gets hit by one of these extremely high energy photons it causes a cascade of different light traveling through the detector that you can then detect in other ways.
Scintillation is just where you hit a molecule and you cause it to vibrate at a different energy that releases a color that you can more directly detect.
Fraser: The whole concept of a mirror or a telescope, that’s bright out.
Pamela: It doesn’t quite work that way with this type of detector. In fact this is one of the problems that we run into with these things. Where you’re not using a normal telescope detector a dish, a mirror or a reflector of some sort it is really hard to focus the light.
Without being able to focus the light you can’t really tell where the light is coming from. We were originally able to figure out that gamma ray bursts come from all different directions in the sky simply because if you had a satellite on one side of the planet Earth and a satellite on the other side of the planet Earth the planet Earth would block out the light from one side of the sky.
Only one of the two detectors would detect it. It was simply the detection not detection planet Earth in the middle that allowed us to figure out which half of the sky gamma rays were originally coming from.
Fraser: Gamma rays will happily go through the side of the spaceship, through the back of the spaceship, through the back of the detector to scintillate the crystal, right?
Pamela: We’ve gotten to the point that we have figured out how to build detectors that can narrow down where on the sky the light is coming from to a reasonable chunk of the sky. Luckily most of the things that give off gamma rays also give off x-rays and we do know how to focus x-rays fairly well.
You say this quarter of the sky had a gamma ray go off. This large but much smaller chunk of the sky had x-rays go off. Then you keep looking at progressively shorter and shorter wavelengths until you can say aha that is the object that is giving off gamma rays.
Fraser: I guess we’ll talk a bit about the Swift observatory a little later in the show but that’s a great example. One of the greatest technical challenges with that instrument was just to be able to have it quickly locate the source of gamma ray burst and slue the whole telescope around to catch it.
There is another mechanism for detecting gamma rays. They’re blocked by atmosphere, right?
Fraser: But there is another way to detect them here on Earth.
Pamela: Right you can also end up with different cascades of particles. You have a gamma ray hit the upper atmosphere. As it hits the atmosphere it interacts with the particles in the atmosphere, the molecules in the atmosphere and causes a chain reaction of what looks in a particle detector like almost a fireworks display.
You see the gamma ray coming in and it ends up breaking off into a variety of lower and lower wavelength bits of light as it cascades through the system. We have detectors like the Whipple observatory that are able to catch this cascade of particles coming down from the single gamma ray, cosmic ray that hit the upper parts of the atmosphere.
Fraser: That is amazingly clever. I just think about that, once again we’re back to dishes but there are these dishes set up that catch this effect. Gamma ray strikes the atmosphere, gets absorbed by the atmosphere but it has so much energy it releases this cascade of particles.
A detector can pick up that cascade of particles and look back and say that had to be a gamma ray of this energy and then actually build up a map of gamma rays. I just think that’s awesome.
Pamela: What’s cool is we have entire systems that are used to try and model backwards to how was it that this ended up getting detected. These are some of the neatest looking detectors out there.
For instance the VERITAS system for Very Energetic Radiation Imaging Telescope Array System if you want a really long-named telescope is made up of a whole bunch of little tiny individual mirrors.
They are then all reflecting back to a single point. We end up with sensitivities to extremely high energy particles just by looking at how they basically make the atmosphere scintillate.
Fraser: That’s amazing. Okay so now we know sort of where they sit on the electromagnetic spectrum. We kind of know what our toolkit is to detect them. What is the source of these high energy gamma rays?
Pamela: We have all sorts of different sources. You can get anything from here on the planet Earth. If you have a radioactive decay you end up with gamma rays. Out in the darkness of space in the dark molecular clouds if you have a cosmic ray flying along a high energy particle of some sort, when it hits the cold gas and dust of these clouds that can end up giving off a gamma ray as this cosmic ray suddenly gets slowed down by its interactions with dust and gas.
Fraser: These cosmic rays are particles accelerated to relativistic velocities. They’re able to impart a lot of energy when they crunch into some gas.
Pamela: In turn all of that energy through the slowing down gets released as light. We see this as a gamma ray background. We also have neutron stars with really strong magnetic fields. When these magnetic fields realign themselves it can give off huge amounts of energy in the form of gamma rays.
In fact there was an event back in the early 2000s where a magnetar – one of these neutron stars with a high magnetic field – right here on our own galaxy rearranged itself.
Even though it was on the other side of the Milky Way’s center from us it was able to overwhelm several different space telescope sensors such that it was just like they had pointed straight at the sun.
It was completely wiped out and had to be reloaded and take new images to see just what was going on on the other side of the galaxy.
Fraser: I guess for awhile there it was like the brightest thing in the sky you just couldn’t see it. We can’t detect gamma rays.
Pamela: This single event gave off as much light as the sun will give off in its entire lifetime.
Fraser: Wow. That’s like a dead star that just had its magnetic field so twisted up that it released that much energy.
Pamela: You can get all sorts of energy trapped in magnetic fields so we also see gamma rays coming off from binary systems where you have a compact object and a regular star in that compact object – either a white dwarf or a neutron star or a black hole – is sucking material off of its nearby neighbor.
Thedisc can also end up with a magnetic field. During this entire process there are lots of different ways that gamma rays can get let off throughout the process.
Fraser: Let’s talk about the big one [Laughter] which is the gamma ray burst.
Pamela: These are cool. There are two different types of gamma ray bursts. They actually have an interesting history of discovery because if it weren’t for quick thinking scientists we might have actually ended up accidentally going to nuclear war over gamma ray bursts.
Fraser: We’ve done a whole show on gamma ray bursts. We’re not going to go too much over that.
Pamela: The highlight of the story is we put into orbit a bunch of satellites to make sure that the Soviet Union abided by its nuclear test ban treaty. The way you detect nuclear explosions is you look for the gamma rays they give off. So we send up these satellites and they start detecting gamma rays.
We didn’t know that there are astronomical sources of gamma rays. This was a sudden new discovery and we had to figure out this is actually coming from somewhere not on the Earth. Now we know that they come in two different varieties.
There are the extremely short bursts, usually fractions of a second to a couple of seconds at most. We think that these are probably caused by a pair of neutron stars or objects including like a neutron star, a black hole or a pair of black holes merging.
Through the merger, through the combination of all that mass coming together in such a small volume of space the energy released during the merger process gives off for a very brief instant more energy than you’re getting from an entire galaxy. Just a normal light in that same moment and these are the short gamma ray bursts.
There are also longer gamma ray bursts that in some cases can last several minutes at a time. This is where you get a giant star. Something like Eta Carinae can do this. If it has the right type of magnetic field, when it dies as a supernova it can funnel the energy from that supernova explosion out the poles of the magnetic field.
That energy if you happen to be along the cone of light, if you happen to be lined up with that magnetic pole, the light can get funneled straight at you. A lot of it is going to be in the form of gamma rays.
Fraser: I just can’t imagine the event that would release the amount of energy of the entire galaxy.
Pamela: It’s really phenomenal. A lot of neat science is coming out of trying to understand these things. The amazing thing is just ten years ago we had no clue what the source of a gamma ray burst was. People were making guesses.
We were guessing that it was a special type of supernova. We were guessing that it might be neutron star, neutron star, black hole, neutron star, black hole black hole mergers going on. Now we’ve actually learned thanks to telescopes like SWIFT that when you follow these things up you get the gamma ray burst.
You get the quickly fading x-ray transient. You get an optical afterglow. You can watch the object change in color very rapidly. Then weeks later you end up with a supernova light curve in the exact same location.
From watching this rapid fade from this amazingly bright object down to normal supernova we’re able to watch the evolution of a dying star in its last gasping breaths. It is creating all the heavy elements and killing anything nearby all at the same time.
Fraser: Gamma ray bursts create pretty phenomenal gamma ray events. Is there anything more powerful? Where do the highest energy gamma rays come from?
Pamela: If you want to have the single most powerful photon that’s probably going to be coming from a black hole in the center of a galaxy that is feeding that black hole. When you have active galactic nuclei, systems like blazars for instance that have powerful jets coming out of them the acceleration that the particles are going through can lead to the release of amazing amounts of gamma rays.
In fact the highest energy photons that we’ve detected so far have been coming out of these high energy active galactic nuclei systems.
Fraser: You can imagine the environment. You’ve got a black hole millions of times the mass of the sun spinning at essentially the limits of relativity close to light speed, its powerful magnetic field wrapped up in this disc of material it is eccreting on to. It is firing out these jets that can be hundreds of thousands of light years long.
A blazar is one of those situations where that jet is pointed right at us. Don’t worry, we’re safe. So, we’re seeing the particles accelerated out of the black hole crashing into gas and whatever. Then we see these monster photons. How powerful can these photons get?
Pamela: So far we’ve been detecting things in the many, many gigaelectron volts. The thing is we don’t know how powerful they can go because our detectors may not be sensitive to the most powerful ones coming towards us.
We’re off in the hundreds of gigaelectron volt land but that’s where we start hitting the limits of our detectors.
Fraser: The highest source of the I guess the highest gamma rays can only be detected by the Earth-based sensors after they’ve passed through the atmosphere. We talked about this before.
The spacecraft can’t grab them. It’s got to be the ground ones. We don’t really have the technology to catch an electron that high.
Pamela: With the ground-based systems we start actually saying that we’re sensitive to the things in the tens of teraelectron volt level. [Laughter] That’s just kind of amazing to think about.
It’s the cascade affect of what happens when the particles interact with our atmosphere that we’re able to see the change in type of radiation. We detect that new form of radiation, Cherenkov radiation as it passes through the atmosphere and gets to the detectors.
Fraser: This is one of those parts of science that have been evolving over fairly recent time. I can remember a time when scientists would say we’re detecting electrons, 10 teraelectron volts and there is no possible way that they could be generated. Yet here they are. [Laughter]
Until they finally figured out that there were super massive black holes which I don’t think had ever occurred to anybody to have a black hole with millions of times the mass of the sun spinning at relativistic philosophies I guess that had never really occurred to people.
That is the source of those high energy photons. No one had ever really thought about that. It’s quite funny it is one of the situations where like with the globular clusters, they’re older than the universe. We don’t know why.
Pamela: [Laughter] And it turns out we just don’t understand stellar evolution as well as we thought we did.
Fraser: Yeah, we knew they’re there. The universe is here. Something is wrong, and we’re not going to worry about that right now. Same deal you know, teraelectron volt – they’re here.
It’s not possible they are to be regenerated but they’re here. Therefore it has to be possible for them to be regenerated and we’ll figure that out later. [Laughter]
Pamela: What’s really amazing is the more we learn about high energy astrophysics – and this is all brand new technology we’re just learning how to do this – the more we learn about high energy astrophysics the more we realize that some of the most interesting things in the universe are basically driven by the hardest to understand astrophysics which is all magnetic fields.
This is one of those fields that can potentially involve the two hardest to understand things which are magnetic fields and dust. Actually you can end up with a high energy particle created in a magnetic field of a neutron star of active galactic nuclei of something. Then it travels through space and interacts with dust and creates this background gamma ray light.
We can end up with all sorts of different things. The high energy universe is something that is extremely exciting and with new telescopes like the Fermi observatory that is now on orbit, formerly known as GLAST observatory is opening entirely new doors to what we’re able to explore.
We’re finally able to start understanding in detail what’s going on with these active galactic nuclei.
Fraser: You mentioned briefly the gamma ray background. What is that?
Pamela: If you survey the entire sky in gamma rays and measure where it is brightest and where is it faintest what we find is there is actually this background everywhere you look of gamma rays coming from the sky.
It is brightest when you start looking out at the disc of the Milky Way. This is where we have high energy particles interacting with the gas and dust within our own galaxy. You also see this diffuse background glow from everywhere you look. This is just a matter of there are gamma rays basically coming in one form or another from anywhere you look on the sky.
Fraser: I guess the last part we haven’t really talked about yet is the cool gear, the technology. [Laughter] What are the missions that have been sent up to help us learn more about gamma ray astronomy?
Pamela: It all started with originally missions that were set up to be looking for nuclear arms. Since then we’ve had a whole series of different balloon-based missions. We were able to look in part for gamma rays using the Solar Maximum Mission. We’ve had things like the Compton Observatory, the High Energy Astronomy Observatory.
Today’s big missions are the High-Energy Transit Explorer – HETE-II that was launched back in 2000 that we’re still benefiting from. We also had Swift which is a little NASA mission that has this amazing array of cameras that allows it to detect things in the gamma ray better locate where they are in the sky using x-ray. It also has optical and ultraviolet so that you can narrow down to know exactly where on the sky this strange high energy object happens to be located.
Fraser: We mentioned earlier that it is really hard to detect the source of gamma rays. It’s quite amazing I think within about a minute or two it can go from detecting a gamma ray burst to homing right in on it and starting to image the afterglow of the burst itself. When you’re dealing with gamma ray bursts it all happens so quickly that you want to get your science recorded as soon as possible.
Pamela: One of the amazing things is all of these missions, Swift which is currently flying, INTEGRAL which is the International Gamma Ray Astrophysics Laboratory, Fermi which used to be the Gamma Ray Large Area Space Telescope all of these missions that are currently in orbit and currentlygamma ray bursts have the ability to send via text message and e-mail alerts out to observers all around the globe who have signed up to be part of these burst alert networks.
They send out the coordinates on the sky and we can start in seconds getting optical follow-up data. This starts some of the biggest telescopes in the world turning to point their spectrographs at these objects so we can find out exactly where they are from their Doppler shift.
We can start finding out what exactly is the composition in some cases. What are all the details of where the light is and isn’t coming out across all the different wavelengths?
Fraser: The high energy astronomy folks are sort of a rapid response bunch. When Swift detects new gamma ray bursts as you said there’s this network of telescopes and messages that are sent out. People drop what they’re doing and shift to the location to try and detect as much of the energy coming out of the gamma ray burst.
It’s quite different than with Hubble and things like that where people book their telescope time months and years in advance. The gamma ray folks, when they see their bursts they have to drop everything. It doesn’t matter what you were looking at, it’s time to look at gamma ray bursts.
Pamela: One of the really amazing things about it is these are people that are multi-wavelength by nature. They detect their object in the gamma ray and then they follow up across the entire electromagnetic spectra. These are people that fluently work in all the different colors that we’ve been talking about for the past several weeks.
It’s not uncommon for someone to specialize in both gamma rays and radio astronomy. They have similar processes such as the active galactic nuclei and the neutron stars can be generating light in both the radio and the gamma rays.
You’re moving from the completely harmless to the completely dangerous. Generating the light through related processes and to fully understand the objects that create the high energy you have to also understand low energy.
Fraser: That would be really interesting I think to have a conversation with someone like that because as you say they live in all these different wavelengths. They have the wavelengths that we see with our eyes but they also are sort of working with their imagination and with their instruments in the other wavelengths.
They can shift between them, this to see that, this wavelength to see this stuff, gamma rays to see that. X-rays over here and I think it must be a lot more natural to them to kind of go up and down in the wavelengths and think of the universe in a wider way. I think that’s pretty neat.
Pamela: These are the smartest people I think in many ways in observational astronomy.
Fraser: Yeah that’s the thing right is to think about that. Maybe we should find someone and talk to them. [Laughter] Okay, I think that was great. So, we’re done. We’re all out of wavelength. I think people want to know is there anymore? There’s no more all just gamma rays from here on out.
Pamela: We’ll be moving on to discussing new and less electromagnetic topics in our next episode.