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Uh, thank you very much for the introduction and the invitation to come and speak here.

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I always like to start off with a nice picture of one of the Veritas telescopes on something that wasn't our best observing night of the year,

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obviously. But we did get a nice shot of the rainbow.

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You can see at the top here, this is the MMT telescope.

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It's the Whipple Observatory in Southern Arizona. And this is the facility I'll be speaking about mostly.

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So I'm going to be talking about exploring the extreme universe with Veritas.

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And we observe the extreme universe by studying in gamma rays.

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This is a point in the talk where I always try to upset any astronomers in the audience by

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telling you that gamma astronomy is by far the most important type of astronomy that there is,

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and I can prove it. So the we typically gamma ray start at the rest mass of an electron.

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So half an MLB and we can observe them now up to 100 TV.

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So 10 to 100 TV gamma rays being produced in astrophysical objects.

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And this we call all of this gamma ray astronomy. But you can see this covers an enormous range of the of the spectrum.

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It's equivalent to shortwave radio, up to low energy X-rays.

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And we tend to lump this all together into gamma ray astronomy that we have to use different techniques to cover different regions of this spectrum,

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just as you do with with other parts of the electromagnetic spectrum.

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I'm mostly going to be talking about Veritas, which operates from a few hundreds of well around 100 TV, up to about 30 TV.

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We're looking for gamma rays. So you first have to think where they come from.

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Gamma rays, of course, are only produced by non thermal processes.

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There's no way you can get an object hot enough that it would emit gamma rays.

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They're produced rather by processes such as inverse Compton scattering.

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So this is where a high energy electron interacts with a lower energy photon and boost that photon up to two higher energies.

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And this is a very efficient mechanism to produce high energy photons, gamma rays,

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because the boosting factor is proportional to the Lorentz factor of the electrons squared.

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So a 500 electron boost the energy of a photon by by ten to the 12, and an infrared photon can become a ten GeV gamma ray.

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So this is a good mechanism to produce gamma rays.

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If you have electrons, if you have hadrons protons, typically then you can produce gamma rays through the process of pion decay.

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So what's happening here is a high energy proton hit some target material in stellar gas.

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And what comes out is more protons and zeros.

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At least that's one result of the interaction. And the PI zero decays immediately into two gamma rays.

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And if you have high energy protons,

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you'll produce high energy gamma rays for both of these processes to take place and to produce high energy gamma rays.

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You need a population of relativistic particles.

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You need a population of particles accelerated up very close to the speed of light.

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So by looking at the universe in gamma rays, we're looking at the extreme universe,

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places in the universe where there are acceleration processes taking place which are able to produce these very energetic particle populations.

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We can produce the gamma rays through inverse Compton scattering or through pion decay.

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Lower energy should have thermal emission from dust and from from stars,

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or perhaps synchrotron radiation through accelerated electrons spiralling in magnetic fields.

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But these are the high energy processes, the two that we're most interested in.

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And one of the things we try to do with gamma ray astronomy is to distinguish between these two processes,

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to discover whether the emission is due to electrons or due to hadrons being accelerated.

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I'll talk more about that later. So how do we get these populations of accelerated particles on the earth?

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You accelerate particles which shows Fermilab.

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You accelerate particles by using electromagnets in tunnels and accelerating the particle particles around in that fashion,

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in space, in the wider universe. There are lots of different.

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Environments where particles can be accelerated. They can be accelerated in the jets produced by active galactic nuclei,

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the supermassive black holes that lie at the heart of a good fraction of galaxies.

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They can be accelerated in the shockwaves formed when a supernova takes place, when a star explodes.

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Or they can be accelerated in regions of very intense electromagnetic fields, such as you'd find around pulsars.

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So there's a variety of environments where you can accelerate particles and produce gamma rays.

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And I'll talk more about some of those later. But I'd like to spend a little bit of time talking about how we detect the gamma rays.

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And the most obvious thing you can do to detect a gamma ray is to send a gamma ray detector up into space,

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which shows you the the large area telescope onboard the Fermi Gallery Space Telescope.

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And this is basically just a big block of silicon with a particle tracker.

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And it's been flying around the earth since 2008.

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And what this does is it sits there. It's about one metre square, and it waits for a gamma ray to hit it.

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The gamma ray pair produces produces an electron positron pair.

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And by following the tracks of those two charged particles, you're able to reconstruct where the gamma ray came from.

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And this is great. It works very well.

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The problem is, if you want to go to high energies, so the LAT works very well up to ten, maybe up to a hundred TV.

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But let's say you want to study gamma rays of one TV, which we know sources exist that can produce such high energy photons.

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Then, for our strongest known gamma ray source in the TV region, you detect we have a flux of around six photons per square per year.

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So if your detector is only one metre square large, you have to be very,

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very patient or you have to come up with some mechanism to increase the effective area.

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So you can either submit a grant proposal to NASA for $1,000,000,000,000 to build a bigger space telescope,

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or you can try and do something a little cleverer. And what we do is to use the atmosphere itself as part of the detector.

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And this allows us to increase the affected area at high energies.

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So when when a gamma ray interacts in the atmosphere, it produces produce an electron positron pair.

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And these can go on and through secondary power production and also brimstone processes.

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You end up with what's called a particle cascade or an air shower coming down through the atmosphere.

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This this shows you. So when you have an ash out in the atmosphere, the particle for one to be gamma ray,

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the maximum number of particles occurs at an altitude of about ten kilometres.

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You get some particles surviving below that altitude.

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So the first obvious thing you can do is to try to detect those particles at ground level.

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And that's what a number of experiments have done. The most recent one is called Hawk.

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The High Altitude Water Cherenkov telescope. And this is it, I think four and a half thousand metres in Mexico at the peak of the recover.

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And these are basically just large tanks of water, two metres tall.

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And when a particle from a gamma ray cascade interacts inside the tank, it produces a flash of light.

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And we're able to record gamma rays in that way.

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So this is good. There are some downsides with this technique.

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It's difficult to discriminate.

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Gamma ray initiated showers from the background of showers initiated by cosmic ray particles hitting the top of the atmosphere,

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which are much more numerous. You also have to build your detector up at four and a half thousand metres, which has some problems.

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But this is quite effective. This works quite well. It doesn't work very well at relatively low energy.

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So below. One TV for example. Simply because the particles don't make it all the way down to the ground.

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So the alternative technique that we use is to use the cherenkov radiation.

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So these charge particles are very high energy. They are moving faster than the speed of light in air.

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You can't go faster than the speed of light in a vacuum, but you can go faster than the speed of light in air, which is see over the refractive index.

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And all of these particles in the shower are moving faster than the speed of light in a when you do that,

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if you have a particle moving rapidly through a transparent medium,

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then the medium radiates and you end up with constructive interference and a wavefront of what's called cherenkov radiation.

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And this is basically blue light. So the analogy here is with a sonic shockwave.

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This is the the light equivalent of a sonic shock shockwave when an object moves faster than the speed of sound in a medium.

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You can see cherenkov radiation around the core of a nuclear reactor, and you see a glowing blue light.

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And this is also being produced by air showers in the night sky.

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So this has been known for a very long time. Blackett in the late forties, early, early fifties,

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did the calculation and realised that something like 1/10000 of all of the light from the night

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sky is being caused by cherenkov radiation due to high energy particles in the atmosphere.

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So he did the calculation and concluded There's no way you can observe this.

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It's just too too small, in effect, to see Jelly.

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And Galbraith took these numbers in one eye and thought about it and realised that the Cherenkov radiation would be there at such low levels,

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but it would occur in very brief flashes associated with these particle cascades.

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So in the early fifties, just 50 miles down the road in Harwell, they built the first Cherenkov telescope.

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And it's very simple.

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It's just a mirror and a photon multiply a tube, a light detector, and you plug that into an oscilloscope, and that's all you need.

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You could go outside tonight and build one of these and it would work fine.

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You'd see about one shower every every minute or so.

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And this is essentially the same technique that we use today with our larger, more sophisticated telescopes.

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The difficulty with just a single photon, multiple tube like this is that you can't record an image of the triangle of light.

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Okay. You can just you can observe a flash, a drink of light, but you can't record an image of the shower in the cherenkov light.

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But it's pretty easy. For that, you need a camera. And this is what people have been doing since the late eighties, is observing these shadows,

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using an array of O2 multiplier tubes which make up a crude camera.

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And the advantage of doing this is that it allows you to look at the properties of.

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The image to reconstruct where the gamma ray came from and to discriminate the

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gamma ray showers from this background of showers produced by cosmic rays,

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by high energy particles.

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So just to show you how that works when a gamma ray and because the atmosphere produces a particle cascade all the way through this cascade,

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cherenkov radiation is being emitted and you end up with a pool of light on the ground with a radius of about 150 metres.

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If you have a telescope lying inside that pool of light and you build a camera the focus of the telescope,

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then you record an image which looks something like this fairly uniform and elliptical.

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A cosmic ray shower would tend to have much more structure.

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The electromagnetic development of the shower makes it fairly compact.

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The the long axis of the shower here corresponds to the long axis of the ellipse.

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And the point of origin of the gamma ray in the sky is going to be somewhere along this this dashed line.

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So if you know, somewhere along that line,

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you can break the degeneracy by building a couple more telescopes now such that they all sit within the same light pool.

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And by intercepting these lines, you're able to say exactly where the gamma ray came from on the sky.

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So in this way, you can start to do astronomy if you're having trouble picturing how that works.

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Astronomers sometimes like to think of it in this way.

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It's similar to the radiance formed by a meteor shower, and in this case, they all point back to the same position on the sky.

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And this position is the direction of motion of the earth through to the tail of the comet or whatever.

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That's what's causing the meteor shower. If you're not an astronomer, you can imagine yourself as Han Solo or Chewbacca.

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And it's just exactly the same thing. The radiance of the things that are zooming past.

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You point back to the air, the direction that you're travelling in or the direction the gamma ray came from in our case.

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So the technique really came to life in the late eighties, early nineties, and that pushed the construction of a few new facilities.

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Veritas is one I work on here in southern Arizona. There's also magic on La Palma in the Canary Islands, and Magic has two large 17 metre telescopes.

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And then there's Hess down here in Namibia, which has four telescopes, about the same size as Veritas.

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And then more recently, they added the largest string telescope in the world, which is a 28 metre diameter.

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It's one of the largest optical telescopes in the world because remember,

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while we use these for gamma ray astronomy, they're essentially optical telescopes collecting blue of light.

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So the reason we were able to build 28 metre diameter dishes is that the mirrors don't need to be very good.

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They don't need to be astronomical quality, so they're relatively cheap.

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So Veritas is the instrument I work with. We constructed it in 2007.

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We completed it. This is a nice picture taken from the hill behind the array.

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It looks a little bit like it's been built in a parking lot,

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and that's because we built it in a parking lot, and that was the only place we could find enough space.

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You want the telescopes to be fairly well separated so that you can get a nice kind of stereoscopic view of the shower,

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but they have to be within this local of a few hundred metres, so they have to be spread out by a hundred metres or so.

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And this was the only place we had room to do that. This is the base camp to an astronomical observatory, the Whipple Observatory.

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So the telescopes are actually relatively low altitude.

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We put them down in the base camp a thousand metres because you don't need to be terribly high altitude for the type of cherenkov work that we do.

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But this does mean we're the only astronomers in the world that sleep up at night,

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up at the telescopes, and then drive down the mountain to observe every evening.

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This is what the array looked like when we when we built it. We subsequently moved this telescope over here in 2009.

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This was the original prototype telescope. And that was the only place we could put the.

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Time we moved it over here to improve the array layout.

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And this has turned out to be useful as well. I'll show you later on.

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This just shows you the the spacing and the telescopes. And this has been built and operated by a collaboration mainly in the US,

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but with come from contributions from Ireland, Canada and Germany and originally the UK.

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There was a large group at Leeds which is where I came from, which doesn't exist anymore.

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Just to tell you a little more about the telescopes. We have four of them.

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Each telescope has a 12 metre diameter dish, so about 100 square metres of mirror area.

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They're exposed to the to the environment.

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So other than build a huge shed to cover the telescopes, we have a programme of continually retouching the mirrors.

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It's like painting the fourth bridge. You just continually re coat the mirrors.

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We have 100 spares that we take off and cycle back on, so each reflector gets completely refloated roughly once every two years.

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And at the focus of each telescope we have a 500 pixel very, very crude camera, but it's very sensitive and it's very fast.

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And so this is made up of focal multiplier tubes, which are very high gain, very rapid photo sensors.

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And we spend our time this is this shows you to a series of images from each of the four cameras of the telescopes.

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So these are maybe not the fastest movies in the world, but they're among the fastest.

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Each frame in these in this movie is about two nanoseconds.

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So you're watching basically a billion events per second frame right here.

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And what you're seeing here are cosmic ray showers. So if you were able to resolve such short time scale phenomena with your eye,

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you'd be able to go out on any evening, look up and you'd see these large showers.

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This gives you the size of the full moon for comparison.

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So these are degrees across and they'd be zipping around in the night sky.

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These be recorded at a rate of about 500 per second, something like that.

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And basically all of these events I'm showing you here a cosmic ray background events.

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So while these are pretty to look at, these are the things we want to get rid of.

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This is our background, the events we like to try and keep look more like this.

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So this shows you a gamma ray event,

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a nice uniform ellipse in each of the four telescopes and the long axis of the ellipse always points back to the same position in the camera.

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So that tells you where the gamma ray came from on the sky. And it works very well.

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We're able to pick out a gamma ray rate of just a few per minute from a background of hundreds per second thousandths per second.

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In fact, we've even been able to by integrating over observations of hundreds of hours,

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we're able to pick out gamma ray signals of less than a photon per hour, something like that.

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So the technique works very well. We're able to reconstruct the position to within about a 10th of a degree, the size of the full moon.

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And we can measure the energy of the shower's sorry, the energy of the incident gamma ray for anything above about 100 g.

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And we observe every night of the year that we can for about 1400 hours in total.

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It's maybe more instructive to show you what an observation looks like.

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So if if you're running the telescopes at Veritas,

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this is what you'd see if you observing the strongest steady gamma ray source in the sky, which is the Crab Nebula.

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After one minute, we have a solid detection of the Crab Nebula.

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These are background fluctuations. This is where the source is on the sky.

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And this is already about five standard deviations above the background.

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After a couple of minutes, it's even clearer and so on.

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And once you get up to 5 minutes, you have ten standard deviations.

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And that's the point on which you are able to measure the gamma ray spectrum of the source fairly accurately.

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And with a very strong source like this, after a 30 minute observation,

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you really start to see this horse come out very clearly against the background.

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So the technique works. The crab was detected in 89 when we turned Veritas on.

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The sky was still pretty much unexplored.

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After we'd been running for a full 26, we detected only three objects the crab and two active galaxies Markarian 41 and Markarian 501.

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After running for ten years now, we've managed to map out the sky a little better.

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So this shows you a projection where the disk of our galaxy is along the equator here.

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The shaded area shows you the region of the sky that we can see at high elevation.

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And we've detected 56, I think now 56 sources.

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But probably more interesting than the the number of sources that we've seen is the fact that they're not they're not all the same type of object.

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So we've detected supernova remnants, pulsars, pulsars and nebulae, binary systems, radio galaxies, starburst galaxies and Blazars.

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I'll tell you a little more about each of these.

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But what this is really telling us is that particle acceleration can occur in lots of different environments in the universe.

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There's a whole astronomy out there waiting for us to to look at and there's plenty of things to see.

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So in 56 sources, I can't I can't cover everything.

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So I like to try and pick out a few that I found find most interesting.

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And to start with, the Crab Nebula itself and the the pulsar at the centre of the Crab Nebula is has historically

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been very interesting and continues to be so we see steady emission from the crab,

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as in the images I showed you earlier.

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And this is believed to be caused by a relativistic wind from the pulsar wind

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of electrons and positrons interacting with the environment around the pulsar.

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These electrons and positrons get accelerated up to much higher energy, and then they inverse Compton,

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boost photons up to gamma ray energies, and we can observe them in that way.

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But this signal is steady and constant. What wasn't expected in the gamma ray region was to be able to observe a pulse signal.

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So for those of you that aren't astronomers, a pulsar is a neutron star.

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The half of a star that's exploded in non supernova, but they're typically rotating very rapidly.

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So you can think of something the mass of the sun, the size of Oxford, spinning around like a kitchen blender.

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And if there are emission regions on something that's spinning,

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you get this lighthouse effect whereby if you're observing the emission region and that smaller than the rest of the object,

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you'll see the beam of emission past your pass, your line of sight.

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So the emission excuse me, so the emission that you observe in this case is going to be pulsed.

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This has been seen across the spectrum for the crab, but prior to Veritas observations, it will always believed that the emission would cut off.

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When you moved into the higher energy regime above about ten GeV, we decided to observe it nevertheless.

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And what we saw is that this doesn't happen. The emission from the pulsar extends out to much higher energies.

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With Veritas, we're able to detect emission above 120 gbps and there's a spectral point here all the way at 400 GMT,

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where prior to this cut off was expected to occur around a few TV.

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And this tells us initially that the the models for high energy emission from the crab pulsar were not were not quite correct.

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Prior to these observations, most of the models suggested that the high energy emission was caused by curvature radiation,

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which is where charged particles move along magnetic field lines close to the surface of the pulsar and produce gamma rays.

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In that way, if this were true, then the high energy gamma rays that we do observe around a few hundred GeV would not be able to escape.

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That's why you. Get this, the sharp cut off may be absorbed before they were able to escape the emission region.

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The fact that we see higher energy gamma rays tells us there must be other processes taking place.

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All the emission must be occurring very far out in the pulsar magnetosphere.

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So this is not totally resolved. There are two or three different competing models which try to explain the higher energy emission from the pulsar.

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So that was an interesting object to look at something I worked quite closely on myself with a gamma ray binary system.

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What we now know to be a gamma ray binary system.

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And this this story started back in 2007 with the discovery of a point like gamma ray source in the plane of the galaxy.

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This was detected by my hats, and all I can really say about it was that it was a point source in the plane of the galaxy.

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They didn't have any more information than that. We went and looked at it with Veritas over the next few years and we didn't detect it.

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So we could either conclude that he was inventing results.

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Veritas was broken or the source was variable.

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We fairly confident that Veritas wasn't broken, so we we assumed that the source is variable.

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We then went to look at it in X-rays and we saw a counterpart in X-rays to this gamma ray source.

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And in X-rays it's also very variable. And by observing in X-rays over four or five years, we were able to observe a repeating period.

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This shows you various different 315 days period overlaid on each other.

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So the final piece in the puzzle, if you like, is to tie this repeating X-ray source to the camera emission.

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And that's what we were able to do in 2012. We made observations very more intensively around the time of the X-ray peak,

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and we see the gamma ray emission peaking at the the same phase of the period.

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So we're fairly confident with this one now that it's a gamma ray binary system,

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a situation something like this where you have a massive star in this case,

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what's known as a B star, a star with ten or 15 times the mass of the sun with a Circumstellar disk around it.

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And orbiting around this, you have a compact object.

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So either a neutron star or possibly a black hole.

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And as the compact object interacts with the material and with the photon fields around, the massive star gamma ray emission is produced in.

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This gamma ray mission is modulated due to the geometry of the orbit and the changing environmental conditions.

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So this is an object that we continue to study and the results get nicer and

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there are various different models to describe this modulation and emission.

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And these are these are nice objects to continue to study.

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And we're hoping to get another one this year because there's a binary pulsar or a pulsar in a binary system like this just been identified,

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which is believed to be on a 40 year orbit. Luckily, the very astronomers in November 2017,

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so the time when the neutron star and the massive star are closest together, should hopefully occur later this year.

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And we'll have a chance to observe this and see if we can see gamma radiation associated with it.

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Another topic I've worked on quite a lot,

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and one of the original motivations for for doing gamma astronomy is to discover the source of the cosmic rays.

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So cosmic rays are a population of relativistic particles that fill our galaxy and their site of origin.

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The place where they're being accelerated is still not completely clearly determined.

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So cosmic rays were detected by Victor Hess back in 1912 when he took a gold leaf electric scope up in a balloon.

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And he saw that this flux of ionising, what he called ionising radiation at the time, actually increased as you went up in altitude.

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This tells you it's not terrestrial. It must be coming from space. Gamma rays can help to to understand where the cosmic rays are coming from.

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Because they're neutral particles. They're produced in the same environment that the cosmic rays are being accelerated,

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but they're neutral particles, and that means they travel straight from the site of origin to the earth.

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Whereas the cosmic rays themselves are deflected by magnetic field.

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So by the time they get to the earth, we've lost all of the information about where they came from.

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So we look for the the sites of cosmic ray acceleration by studying the universe and gamma

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rays in one place that's been proposed as the acceleration site is supernova remnants.

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So this is the expanding blast waves of stars that have gone supernova.

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And one object we've been looking at quite intensively is shown here.

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This is ice four, four, three, which is also known as the jellyfish nebula, because apparently it's supposed to look like a jellyfish.

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And this is the remains of a star that exploded somewhere between three and 30,000 years ago.

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And it's quite a complex environment.

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If you keep this picture in your head, you can see the star that caused the explosion would be somewhere in the centre here.

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And you can see an expanding shell out to this side, a larger shell out to this side, and then a dark band cutting across the rim here.

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And the way this is usually explained is that the smaller shell is expanding into a denser environment.

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So it's expanding more more slowly. The larger shell is expanding into a more diffuse environment.

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So it expands more rapidly.

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And cutting across all of this and even through the remnant is a cloud of cold molecular gas which blocks the optical emission.

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So we can't see it. But this dark cloud of molecular gas, much more dense than the ambient interstellar medium,

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actually forms a really good target for accelerated cosmic rays to interact with and to produce gamma rays.

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So while in the optical, the remnant looks like this nice and bright over here and over here with the dark band,

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you might expect the gamma ray image to look almost like a negative of this.

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So it would be brighter. But you have lots of target material for the cosmic rays to to interact with.

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So we observed with their attacks that one and our latest images do indeed look pretty much like that.

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We're now able to resolve all of the remnants if we place it on top of the optical picture here we see the brightest

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parts of the emission here seems to be coming from the region where you have the densest amount of material.

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And just to put this in context, this is the size of the full moon.

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And if you were able to go out tonight for today and observe the sky with gamma rays,

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this will be one of the brightest objects you'd be able to see sitting there resolved the remains of the star that exploded 10,000 years ago.

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We see objects outside of our galaxy as well. And luckily, NASA's getting excited about these.

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So they make nice movies for us.

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And what's happening here is you have a supermassive black hole at the centre of an active galaxy which produces a jet of material.

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And if that jet is directed the long line of sight, then we are able to see gamma ray emission associated with this.

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And the gamma ray emission in this case can be very rapidly variable.

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You can have something associated with the core of a galaxy that's varying on timescales of minutes or seconds.

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These are most common source classes that we observe.

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They can be very bright, much brighter than the crab, for example, which is the brightest steady source on the sky.

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These can reach fluxes of five or ten times the crab.

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We see active galaxies if they're close and they have a jet.

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But the jet is misaligned with our line of sight. So the jet doesn't necessarily have to be pointing towards us.

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If the if the galaxy is very close, then we can see gamma ray emission also associated with these objects.

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These are interesting because if you're able to resolve because the galaxies are close,

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you're able to resolve the jet in X-rays and radio and possibly try to correlate changes in structure of the jet with the flux of gamma ray emission.

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So we can't actually resolve a galaxy and its jets and gamma rays or angular resolution is not good enough.

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But we are able to correlate the integrated flux of galaxies with the structure seen in other wavelengths,

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and we can observe starburst galaxies as well outside of our own galaxy.

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These are galaxies like our own. But with maybe 30 times the rate of supernova explosions producing many more cosmic rays in a denser environment,

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so they just glow generally in gamma rays.

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I think I'll skip that because I'm getting a little slow, a little short on time, and I'd like to leave a few minutes to talk about the future.

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So what Veritas and Hess and Magic of have all shown us is that there's plenty of things to look at.

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There's lots of environments where particle acceleration takes place and there's a whole astronomy out there waiting to be done.

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The Veritas catalogue has about 56 sources.

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The entire catalogue for the gamma ray sky at these energies is only about 150 sources.

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So there's very likely much a lot of stuff there that we just haven't had the sensitivity to observe yet.

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So what we need is a better telescope. And this is where the Cherenkov telescope array comes in, which Garrett and myself are both working on.

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And CTI is planned to be a large telescope array using exactly the same technique and methodologies, Veritas and the others.

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But on a much larger scale. You can see a mock up of the array here.

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And the idea is one difference you'll see with Veritas is that we have telescopes of different sizes.

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So here we have four large telescopes.

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These are planned to be about 23 metres in diameter, surrounded by a cluster of 20 to 25 medium sized telescopes about the size of Veritas,

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12 metres in diameter, and then a wider, more spread out array of smaller telescopes, each with about four metres diameter.

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And the idea here is to expand the dynamic range of your observations.

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As I mentioned at the start, gamma rays cover an enormous range of the electromagnetic spectrum.

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And with a design like this, you're able to use the large telescopes to target lower energies below 100 GeV.

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The medium sized telescopes target energies around one TV, and then the larger telescopes spread out over a much larger area.

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Because the flux of gamma rays is so low at the highest energies, they're able to give you sensitivity up in the 10 to 100 TV regime.

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So Ccta, with a much larger number of telescopes and three different sizes of telescopes,

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should be able to give you a much larger range and a factor of ten sensitivity improvement on the on the current detectors.

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It's planned to be two observatories, one in the south.

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The site's been set to be in Chile of the European Southern Observatory and one in the north, which is going to be in La Palma.

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So this is all under development at the moment.

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The telescope designs are being put together.

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You have the large telescopes, the medium sized telescopes and the small telescopes, and there are different ideas about the best way to build these.

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And one particular new idea that's being implemented is the idea of using a dual mirror system.

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And there are a few different advantages to this.

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One of the main ones is that you're able to reduce the size of your camera package and that allows you to fit.

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I'll show you a picture at the end that shows you what that allows to do.

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So coming back to Veritas, if you remember, we moved one telescope from here over here.

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So this gives us a nice empty pad there to build a new prototype on for the the CTI experiment.

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And that's what we're doing. This is a mock-up of it, but this is a photograph taken yesterday.

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And you can see we have the structure built of one of these medium sized dual mirror telescopes.

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So the mirrors on yet when they're on, this is going to be nine metres diameter.

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They'll be a secondary mirror here and the camera will be hidden behind the mirror with the light reflecting back onto it.

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So we're quite excited about this, unfortunately. Garrett and Andrea, his student,

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and the rest of the group beat us to it a little bit because they managed to build a dual mirror telescope before we did.

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And this shows you the Gamma Ray Cherenkov telescope, which they constructed, which is a design for one of the small size telescopes of CTI.

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So this is four metre diameter in total, but with a dual mirror system.

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And here you see the camera, the the focus of the secondary mirror.

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So this was inaugurated in Paris earlier this year, I guess, and operating in Paris,

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which is not the best observing place in the world on the full moon.

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They were able to record first light and this shows you the cherenkov image of a cosmic ray shower detected using a dual mirror telescope system.

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And here you can see the Veritas field of view on this camera would be about this big something like this.

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So the advantage of this dual mirror system is that it gives you a much larger field of view with much finer pixelation.

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So you get much, much better images, much more information in the images, and you're able to cover more of the sky less expensively as well.

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So I'll finish there. High energy gamma rays providing us a new view to the extreme universe.

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We know the technique works from the current generation of instruments,

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and the very first results already allow us to do lots of interesting astrophysics.

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But CTA is coming and is going to improve things.

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And just as a taster, this is a simulation of the plane of our galaxy with a zoom on the inner 20 degrees or so,

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showing you the kind of angular resolution you can expect to get and the sheer number and variety of sources that you might expect to be able to see.

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Scott Then you got any questions?