University Place

>> Welcome, everyone, to Wednesday Nite at the Lab. I'm Tom Zinnen. I work here at the UW Madison Biotechnology Center. I also work for UW Extension Cooperative Extension, and on behalf of those folks and our other co-organizers, Wisconsin Public Television, the Wisconsin Alumni Association, and the UW Madison Science Alliance, thanks again for coming to Wednesday Nite at the Lab. We do this every Wednesday night, 50 times a year. Tonight, it's my pleasure to welcome back to Wednesday Nite at the Lab Stefan Westerhoff. He's a professor in physics. He's here to talk to us about the new HAWC gamma ray detector in Mexico. When I was working at the National Science Foundation a few years ago and writing speeches for the director of NSF, the director at the time, Arden Bement, said to me on several occasions that he saw his job as director to insure that US scientists could work with the best people, with the best equipment, at the best places in the world. I think this project tonight illustrates that commitment. It's in Mexico. It's an international project, but it's a great way to see how NSF views its role in US science and in international science. I think it's going to be a fascinating story. Looks like a whole lot of grain silos there in Mexico. We're going to get to see that it's not full of grain but full of water. Please join me in welcoming Stefan Westerhoff to Wednesday Nite at the Lab.

APPLAUSE

>> Thank you very much for the introduction. I'm glad to be back here. The last time I gave a talk, it was four years ago, and at that time I told you about an experiment at the South Pole and in Argentina. And today I will talk about an experiment in Mexico. So this is a view of the experiment. It's called the High-Altitude Water Cherenkov Observatory, or HAWC. I will convince you in this talk that this is actually a telescope. Although, it certainly doesn't look like one. What you see in the back there is Pico de Orizaba, which is one of the highest mountains in North America. It's over 5,000 meters high. The experiment itself is at 4,100 meters, and I'll explain doing this talk why it has to be at such a high altitude. The black that you see in the back is actually an old cold lava stream. So Pico de Orizaba is what they call a dormant volcano. The last eruption was 500 years ago, and we hope that there is nothing coming up in the near future. You shouldn't be worried about the lava steam going directly towards the experiment. There is a valley in between that you can't see on this slide. So, here is, I would like to start with just saying what HAWC is, if you want to take that way. HAWC is just a plan or array of 300 detectors that you see over there, each of which is instrumented with 185,000 liters of water. It's located at 4,100 meters altitude. It's about 14,000 feet or so. And the reason why we built this is, the primary goal of the experiment is to detect gamma rays from astrophysical sources, galactic and extragalactic sources. Here's a little closeup. This is one of my favorite pictures. It looks kind of a little bit bizarre. It doesn't look like a physics experiment. It's a good time to talk about this because a week from today, from Friday, last Friday, we had the inauguration of this experiment. We have been taking data over the last one and a half years or so, but the inauguration with the NSF director happened last Friday. So, here's the outline of my talk. I started to talk a little bit about why we are doing this, the science motivation behind this. What is TeV gamma ray astronomy. What is TeV for that matter. Then I will talk about cosmic rays and their sources. Then I will go into the detection techniques and talk about air shows in general and HAWC Observatory. And at the end, we can look at a few early results from HAWC because, as I said, we have been taking data and we have already published a few results. The type of physics that HAWC is doing usually falls under the area of particle astrophysics. So if you think about classical astronomy, that's typically done using the electromagnetic spectrum from radio all the way to X-ray. Particle astrophysics deals with energies where we think about these waves more as particles. Gamma ray astronomy, for example, deals with photons of light particles that have an energy that's about 10 to the 10 times larger than optical light. So it's the very high energy end of the electromagnetic spectrum. And just for completeness, we do particle astrophysics also with other types of particles. So not just parts of the electromagnetic spectrum but also other particles. For example, cosmic rays. These are protons and other nuclei. They can have an energy all the way out to several joule. That's like a tennis ball hitting you. And these are the highest energy particles observed in the universe, and we have no idea where they're coming from. And, of course, as you know, neutrinos, they're sort of tightly connected to cosmic rays and the sources, but they're neutral and come to us in a straight line unlike cosmic rays. So, in my last talk four years ago, I talked about the cosmic rays and neutrinos. This time I'll concentrate on the gamma ray part of this list. So, here's the electromagnetic spectrum, and you can see that this is on a logarithmic scale. Increasing wavelength in that direction. Frequency and energy go to the left here. You can see that the optical part is really only a very, very tiny part of the electromagnetic spectrum. Wavelength is from about 400 nanometers, 10 to the minus nine meters, to 700 nanometers. That's visible spectrum. And, of course optical astronomy works in that area. But astronomy over the years, decades, has expanded into other energy ranges. For example, of course, into the long waves. So this would be radio astronomy on the right there. But also into X-rays, higher energy particles are going to the left in the direction of higher energy. And if you think about TeV gamma ray astronomy, that means we are going several orders of magnitude higher in energy than X-rays, all the way up to the left on this plot. So the highest energy gamma rays, gamma ray waves, or photons that we can detect. Before I go into that, let me explain what I mean by electron volt. That's a unit that we typically use of the energy. It's kind of inconvenient to use joule because these particles have very little energy so you don't want to carry 10 to the minus 19 joule around. So the unit of energy in this area is the electron volt, and what that is is, if you think about an electron that goes through a potential difference of one volt, as you can see in the little plot over here, that's the energy that the particle would gain is one electron volt. And that's about 10 to minus 19, 1.6 times 10 to the minus 19 joules, or a very small number. On the other hand, think about this is the energy of a single, tiny, little particle. So when we talk about TeV electron volts, or 10 to the 12th electron volts, that's quite a substantial amount of energy. I think it's about the energy of a mosquito hitting you.

LAUGHTER

But I haven't checked that, but that's what I've heard. So, now, astronomy has always worked in different wavelengths and different energies, and the most familiar one, of course, is the optical wavelength. But, as you can see over here, there's an optical picture of the sky. This is plotted in what we call galactic coordinates. So the galactic plane is the center plane that you see, the horizontal plane. And in the optical, which is typically energies of about an electron volt, that is about the range of energies, two to three electron volts for the optical light, what you see on this map is basically stars and low density gas. So at this energy, what you learn from a map like that is where the stars, where is gas that emits at optical energies. Now, you can go to lower energies. For example, on the upper left, that would be radio. That's about a milli-electron volts, two milli-electron volts. So it's three orders of magnitude down in energy. And you can see, once again, like you did on the other plot, you can see the galactic plane. And you can also see here the galactic plane very prominently in the radio. And what the radio map tells us is not where the stars are, also that, but of course also where ionized interstellar gas is. So different energy, you learn different things about the universe. Another energy range that people have been look at is the infrared, on the lower right. Again, you see the galactic plane very prominently. This is at energies of about 0.01 to 2 electron volts. And what you can see there is mainly dust. Infrared is heat. You see dust that is warmed by starlight, and you see star-forming regions. So, again, at different energy ranges you learn different things. This is going to lower energy. We can also go to higher energy. So it will be shown on the upper left here. That's X-ray energies. So here we are talking about a thousand electron volts. So three orders of magnitude going to higher energies, kiloelectron volt. And what you can see there is mainly hot gas. So the X-ray map tells you about where the hot gas is. And then, going all the way up to gamma ray energies, which is what we'll talk about today, you go to 100 meV. This is a picture of gamma rays, the universe in gamma rays. Again, you see the galactic plane very prominently. And what you see there mostly is collisions of cosmic rays with nuclei and interstellar clouds. And I will explain later what that means. So what you can see here is mainly gamma rays produced by interactions of high energy cosmic rays with stuff in the universe. So, these images that I just showed were done by different instruments. Some of them are Earth bound; some of them are space bound. And the reason for this is, of course, that the atmosphere is not transparent to all wavelength. What you can see over here is the atmospheric opacity as a function of wavelength. And you can see that wherever this sort of greenish thing goes to 100%, that means the atmosphere is completely opaque. So you can see that in the visible part over there, the visible wavelength, we have a little bit of a chance because the atmosphere is not completely opaque. It's actually quite transparent. It's, of course, not a coincidence that we can actually see, our eyes can see in optical wavelengths. They're obviously matched here. If our eyes could only see gamma rays, as you can see, we wouldn't see anything because gamma rays won't make it through the atmosphere. So the atmosphere is completely opaque to gamma rays. So you have to go up above the atmosphere to measure them. You can also see that the radio is very good wavelengths to do astronomy from the ground because at radio wavelengths the atmosphere is completely transparent. So, apart from the optical, it would be nice if you could see radio waves. But on the other hand, if you look and see one meter high, it would be one meter big, so that's probably not a good idea. But you can see that for our energies that we are talking about here, at gamma ray energies, the atmosphere is actually opaque. So we'll see what we have to do there. One thing one can do is, of course, send a satellite up. This is the Fermi Gamma-ray Telescope. It has been up for a couple of years now, and it has made these wonderful cosmic rays maps that I just showed you. Here it is again. So, this gamma ray telescope has made this nice map of photons above one GeV. And, again, you see very prominently the galactic plane. Okay, you can also see some points. Here, for example, here on the Crab Nebula over here. So apart from this diffusion in the plane, you see points, and off the galactic plane over here, you see extragalactic point sources of high energy gamma rays. So we have about a thousand gamma sources, 2,000 gamma ray sources seen by this telescope at GeV energies. And if you ask what these sources are, well, most gamma rays that you see on this plot originate, as I said, in collisions of cosmic rays with nuclei and interstellar clouds. That's where this whole diffuse emission on the plane comes from. So the Milky Way in this plot is a diffused source of gamma ray light. But superimposed on that are, as I said before, point sources that can be galactic. So these point sources over here in the galactic plane, the Crab Nebula and so on, and away from the galactic plane where they are extragalactic sources. So if you subtract the diffuse flux and look at where the sources are, here's the map of these 2,000 sources that have been seen by this instrument. And you can see that the galactic plane is pretty populated, but you also have a lot of objects, extragalactic objects of the galactic plane. So this is a very successful instrument, obviously. It has sort of opened this window of GeV in astronomy. The question is, of course, now, can we go even higher in energies? So what about TeV energies? Here is now the TeV sky at 10 to the 12th electron volts. And you can see still the same thing, more or less. The galactic plane is visible but you also see that this map is rather sparse. Not that many sources out there. There are several reasons for that. So the total number of sources of these energies, the TeV catalog of, essentially, today, is about 160 sources, 100 of them galactic so they are in this plane, and the other ones extragalactic. And this map, unlike the other maps that I showed you, is not a unbiased map of the entire sky. This map, for reasons that I will go into in a couple of minutes, is strongly biased, and that has to do with the instruments that have detected the sources that you see on this map. And to make an unbiased map of the TeV sky, that's one of the main goals of the HAWC instrument. So, what are these sources? By the way, I should talk a little bit about that. The galactic sources that you see on that plot and GeV and TeV energies are mostly supernova remnants. So these are the leftovers of supernovae that expanded into space a couple hundred or thousand years ago. The extragalactic sources that you see off the galactic plane, many of them are what we call active galactic nuclei. These are supermassive black holes. About 10 to the eighth solar masses in the center there. Right here in the center is a supermassive black hole. Then material from the surrounding accretes onto this supermassive black hole in an accretion disk that has to do with angular momentum. So we have this spiraling disk. And then for reasons that are not yet fully understood, two jets come out of these that you can see over here. And we think that the acceleration of cosmic rays and the projection of gamma rays that we will see in HAWC is produced in these jets where blobs of material go along those jets and magnetic fields play in. So, one of the things that you can ask is whether these sources that we see at the very highest energies, TeV and GeV energies, are these different sources than those we see at other wavelengths? And the answer is no. What you can see over here, that's a little bit of a busy plot, but I'll explain what it is. This is the flux of a particular source that's Tigris Supernova remnant, which went off in 1572. And this object is observed, as you can see there, over 15 orders of magnitude in frequency. So what you can see on the left side is it has been measured in radio wavelengths. And then we go all the way up to X-ray. Then Fermi lattice is this telescope, the space telescope that I just showed, and VERITAS is one of the TeV instruments. So it's the same source. It has been observed over more than 15 orders of magnitude in energy. So this object puts out radiation and particles over 15 orders of magnitude in energy. This is a galactic object. Here's an extragalactic one. You can see kind of the same thing. You have radio emission, optical emission from this object. X-ray emission, GeV and TeV emission. So, again, 15 orders of magnitude in frequency or energy is the output of this object. And the way this is plotted on this plot, it tells you that, so the scale tells you a little bit about the energy output. And you can see over here that, in fact, most of the energy output of this object is in the GeV to TeV range. So that's our energy range for HAWC. So, there's another interesting thing about these objects, and that is they are highly variable. What you can see over here is the flux from one of these extragalactic objects, IC310, observed with a telescope that I will explain later, as a function of time. And the little bar over there tells you what 10 minutes is. And you can see over here that this object flares on the order of 10 minutes by a factor of, I don't know what it is, 30 or so in flux. So they're highly variable. They flare to some high flux and then they go back and then they go up again and flare. So, very irregularly they have these huge flares, and in order to understand how these objects work, we need to catch them, these flares. So, to summarize that little part, why, then, do we study the TeV band? Well, TeV gamma rays are the highest energy gamma rays observed so far. Clearly, the energy frontier of astronomy. TeV sources emit radiation of more than 15 orders of magnitude in energy. That's awesome. So from radio to TeV, we need to understand where that comes from. And, again, historically, whenever astronomy has opened a new window, for example, going into the radio frequencies or into the X-ray frequencies, we have discovered different things, as I showed. If you look at the universe at different wavelengths, X-ray, radio, optical, you learn different things. And the thing that we think we're going to learn about the TeV window or the TeV map is where the sources of cosmic rays are. And that, in fact, is a problem I talked about when I was here four years ago. So obviously we haven't solved it in four years. But it's a 100-year-old problem and mystery. So let me talk a little bit about that. That is one of the fundamental things we are trying to do with HAWC. So cosmic rays were discovered in 1912. So cosmic rays are particles that rain onto the Earth from outer space at very high rate over many, many orders of magnitude in energy. And around 1910, those were the hay days of radioactivity, study of radioactive materials, those were typically done with these things called electroscopes that you charge, and then if there's a radioactive material around, they discharge over time. But people noticed that if you leave them alone in the lab without any radioactive material around, they also discharge slowly. So the idea was, well, maybe the Earth itself is radioactive and slowly discharges electroscopes. And then this guy in the balloon over there, Victor Hess, decided if that's the case, if I go up in a balloon with one of these electroscopes, the discharge rate should go down because I'm going away from Earth, which is the radioactive source. So that's why he went up in the balloon here in Austria. You see many schoolkids around there. Must have been quite a party. And this is his log book. This is what he measured. You can't see it here, but he also plotted it, so here we go. He showed that as a function of altitude, the radiation actually goes up not down. So, you're not going away from the radioactive source; you're going towards it. So, he showed that since the radiation level increases with altitude, that means these particles must come from outer space. That's why he called them cosmic rays. This is a picture of him later. He immigrated to New York, and that's later in life still, I guess, looking at cosmic rays. This is the cosmic ray energy spectrum. So, again, these are charged particles, not gamma rays. But they also go over many, many orders of magnitude in energy. They go all the way from 10 to the, well, 12th electron volts here, to 10 to the 20. 10 to the 20, remember, is the energy range where cosmic ray experiments like the Pierre Auger Observatory in Argentina work. So, the sort of interesting thing about this plot, which, again, shows the flux as a function of energy, you see that with energy the flux drops pretty quickly so at the very highest energies we have very few particles. At the very left end of this plot, cosmic rays typically come from the sun, but for most of the energy range that you see in this plot, we have no idea where these particles come from. So it's completely unknown where they come from. There are some ideas where they might come from. For example, this part, shown in yellow here, we think these are cosmic rays that are produced in galactic sources. For example, these supernova remnants that I just showed. At the very high energy end over there, galactic sources just cannot produce such high energy particles and you cannot confine them in the galaxy. So they probably come from extragalactic sources. So the red part over there is probably extragalactic in origin, the yellow part galactic in origin, but, again, there's not direct evidence for any of these. We have no idea where these particles come from. So, the idea that the galactic cosmic rays come from supernovae remnants is pretty old. It comes from Fritz Zwicky, a Swiss astronomer we can see over here. I think he was, actually, a very nice guy.

LAUGHTER

In spite of this photograph. Here is the Crab Nebula. That's one of the supernova remnants. This went off in the year 1054, seen by Chinese astronomers. The Crab Nebula is one of those permanent sources that emit over 15 orders of magnitude. So it's of a standard candle for an instrument like HAWC. When you build an instrument like HAWC, it's the first source you have to see because it's the strongest source in gamma rays. So, extragalactic sources I already mentioned. Active galactic nuclei, these supermassive black holes with the accretion disk and the jet, they are favorite sources because, energetically, they can do this. They can provide the flux of extragalactic cosmic rays. You see one of them here --. And you can actually really see these two jet structures going out and going into the interstellar medium and hitting stuff there and producing these huge clouds. So, again, they consist of a supermassive black hole at the center, an accretion disk, and two jets with shocks that move outward. So, well, if you want to know where cosmic rays come from, why don't we just measure where they come from and look at the sky and make a map and see where they come from? Well, there is a problem. The cosmic rays are charged and the universe is full of magnetic fields, and, as you know, charged particles in the magnetic field, they get bent. So we have to go all the way to 10 to the 19 electron volts. A very high energy end of that spectrum that I showed you. Four of the deflections will be so small that on Earth they would point back to their sources. So at any other energies, at any lower energy than that, they are completely scramble the magnetic fields, and that's shown over here. You have your cosmic accelerator there, the cosmic ray is on its way to Earth, but somewhere there's an interstellar, intergalactic magnetic field and will scramble the arrival directions. So the arrival directions of cosmic rays on Earth is essentially isotropic. That's not quite true. I will get back to that. But it's mostly isotropic. So we cannot point back to these objects. However, luckily, when the cosmic rays are accelerated in these supernova remnants or in these active galactic nuclei and get out, they hit other stuff that's around. So they leave their sources freshly accelerated, but there's stuff around. There are photons around, there's material around, so they dump into stuff. So these objects are mainly also beam dumps. So the beam of cosmic rays comes out, as you can see on the right-hand side over here. So this is the accelerator. The proton, for example, the cosmic ray particle comes out, but then it dumps into stuff. For example, this gamma over here, and all kinds of particles are produced, and the decay products of these particles are gamma rays and neutrinos. So, high energy gamma rays and neutrinos.

And that's the good news

both of them are neutral so they can come to us in a straight line. So we can actually look where they come from, they can map and identify the sources. And these are also, then, the sources of cosmic rays because that's how the gamma rays and the neutrinos are produced. So the energy, as you see here, escaping the sources disturbed among cosmic rays, gamma rays, and neutrinos. And, in fact, instruments all over the world, which I'll show in a second, are looking for these three types of particles So, they come each with their strengths and their disadvantages. So, cosmic rays, first of all, have a very large flux. That's good. We can easily get statistics. However, they're deflected by magnetic fields so that's not good. The gamma rays are undeflected. That's good. The problem with gamma rays is they often get stuck. So they can get stuck on the way to us, for example, in the microwave background that's the cosmic microwave background photons that these high energy photons can sort of run into and then they produce other particles and they're essentially gone. So, really, we cannot see very far in gamma rays because the universe is not opaque for gamma rays. I also mentioned that they are a little ambiguous in origin. I'll get back to that. Neutrinos are really the stars here. They're undeflected. They don't get stuck. So they could come to us. However, as you probably remember when you heard talks about IceCube, they're a little bit difficult to observe. So you have to build cubic kilometer detectors at the South Pole to actually see them. So that's their disadvantages. So, again, cosmic rays are deflected by magnetic fields. We haven't had any luck so far in finding the sources of cosmic rays. Neutrinos are the smoking gun for cosmic ray acceleration, but they're also difficult to detect. We have detected astrophysical neutrinos in IceCube, but no sources have been identified so far. That's the advantage of gamma rays. As I showed before, we have 160 sources of TeV gamma rays. So, they are really a very nice particle that you can easily detect and identify sources of where they're coming from. But the problem with that is, unlike the neutrinos, they are not smoking guns. So if you see a neutrino from a source, you know that cosmic rays were accelerated in that source and that this source is one of the long sought sources of cosmic rays. If you see a gamma ray, there are other ways of producing gamma rays. So these are not smoking guns for cosmic array acceleration. I will not go into the details how we can produce them, but there are other ways to produce high energy gamma rays in these sources that theorists think are possible. So they're not smoking guns. And, again, they can be absorbed, so we can only see to a certain distance in the universe before these particles get stuck. So, these are their weaknesses, but, on the bright side, we have sources. That's a good thing. The idea how to actually find the sources of cosmic rays with these three particles and their weaknesses and their advantages is, of course, we have to look for all of them and collect sources that emit in all of these. And here is a sort of a set of instruments working worldwide on finding neutrinos, cosmic rays directly or gamma rays. The sort of bluish ones are gamma rays. So that's HAWC in Mexico, Hess in Namibia, MAGIC on the Canary Islands, and VERITAS in Arizona. The sort of yellowish ones are cosmic radio detectors, and then there's IceCube at the South Pole looking for neutrinos. So, all these instruments look for either cosmic rays, neutrinos, or gamma rays, but we think that they're all coming from the same sources. So, let's go a little bit into how we detect TeV gamma rays. On one of the first slides, I told you that atmosphere is completely opaque for TeV gamma rays. So why would we do an experiment underneath the atmosphere in Mexico to detect TeV gamma rays? That's not going to work. So, here's the thing about gamma rays. First of all, the flux of TeV gamma rays is more than six orders of magnitude smaller than at GeV energy. So you can see you go from 10 to the ninth electron volts to 10 to the 12th electron volts. You're going up three orders of magnitude in energy, but the flux drops more than two, actually almost three orders of magnitude for every order of magnitude you go up in energy. So, going from GeV, where the Fermi Satellite worked above the atmosphere, to TeV energies you drop by six order of magnitude in flux. So, what you really need is an instrument that has a large area. So at least a soccer field size. So, remember what size satellites we have. Typically about a square meter. So you cannot send a soccer field size satellite up into the atmosphere. That's not going to work. So the experiment has to be Earthbound. But the atmosphere is opaque, 100% opaque to gamma rays. However, here's the thing. The gamma ray comes into the atmosphere, and it will interact with air molecules. And what's going to happen then is it develops a casket of secondary particles which we call an air shower. So, in principle, the atmosphere is a gigantic detector, colorimeter, where the particle loses all its energies by producing other particles. This is a computer simulation of what that looks like. This is the incoming particle. It interacts at some point with nitrogen or oxygen, produces other particles. These particles are very energetic, of course. They produce other particles, which produce other particles and other particles and that's what you get. That's what's an air shower. It's like 10 to the five to 10 to the 7th particles that are produced by the incoming particle. So, you have to reconstruct the property of that particle that came in, which you can barely see here anymore, from these particles. By the way, the different colors are different particles, and you can see that the shower sort of reaches a maximum and then it dies out. The only particles that make it all the way to the ground are the green ones. These are muons. The muon flux at sea level is about one per square centimeter per minute from all these particles. So we can detect gamma rays then in two different ways. There are two methods that are currently successfully used to detect gamma rays. You can see the shower over there in the atmosphere. You can either detect the particles that actually hit the ground and get the information from the incoming primary particle from those particles that hit the ground. So you build a huge detector on the ground, which is what HAWC is. Or you detect these by the Cherenkov light they produce, and I will talk about that in a second. So, while going through the atmosphere, these showers produce a peculiar type of light, and if you have a light detector and watch these showers, you can see this light. So both techniques, I use Cherenkov light and photo multiplier tubes, and I will go into these in the next few slides. So, the Cherenkov light. What is that? That's sort of the optical equivalent of a supersonic boost. That is the supersonic boost that happens if an object moves faster than the speed of sound. But objects can also move faster than the speed of light, in a medium. Not faster than the speed of light in vacuum. Nothing can move faster than the speed of light in vacuum. But, for example, in water, the speed of light is much less than the speed of light in vacuum. So particles can go faster than the speed of light in a medium. And if they do, they produce that kind of supersonic boost just in the optical. And that's called Cherenkov light after the Russian physicist who discovered it. So, Cherenkov light is produced when a particle travels to a medium, for example air, water, at a speed faster than the speed of light in that medium. So, Cherenkov light is emitted in the forward direction. So the black line over here is the particle trajectory, and the emission angle of the Cherenkov light depends on the medium. So the index of refraction of the medium determines the opening angle. You can see over here that air has an index of refraction which is pretty close to one. So the Cherenkov angle is very, very small, only about a degree. So the Cherenkov light is emitted continuously while the relativistic particle goes over here along the track. In water, which has an index of refraction of four over three, 1.333, the Cherenkov angle is much bigger, so these particles get emitted at a much larger angle. So these two instruments that use these two effects are air Cherenkov detectors and water Cherenkov detectors. I'll talk about air Cherenkov detectors first, and then HAWC is a water Cherenkov detector so I'll talk about it after that. So this, by the way, is, if you don't believe me that Cherenkov light exists, this is a nuclear reactor. High energy particles come out, go through the water, and Cherenkov light is kind of bluish and then the UV. So you can see it in reactors. Don't do that, but this is a photograph taken.

LAUGHTER

And that's the good news

Taken of one of these reactors. So, we need to detect that light too. So, in order to detect light quickly, very fast instruments to detect light are photo multiplier tubes, or PMTs. What they are are essentially light bulbs in reverse. You will see one, the HAWC ones over there. What happens is that the light particle, Cherenkov light particle will hit that glass sphere. There's a material coated onto that sphere that easily emits electrons. So if a photon comes in, it can easily knock out an electron. That electron gets accelerated by electric fields towards the center of that tube, and it hits another plate which also easily gives out electrons. So this electron being accelerated now being high energy hits that other plate and knocks out more electrons. And if you repeat the process a couple of times, you can make a large number of electrons out of one single photon that came in. So we can actually measure the current that comes out of this tube. So it's a light bulb in reverse. You don't put current in to get light out; you have light coming in and get current out. And these are sort of classical instruments that detect light at very, very high speed, which is essential for astronomy here. So, these air Cherenkov telescopes, the way they work is you have a far away sources, the air shower comes into the atmosphere, and this Cherenkov light is produced along the track, and then you have this type of instrument that points towards the source and detects this Cherenkov light. So you see these little mirrors over there, they reflect back into a photo nucleic camera that sits at the focal point over here. So they can see this sort of light thing projected onto this camera and then can reconstruct the energy in the direction of that light particle. So, these instruments are very, very sensitive to point sources, and they have created most of these 160 sources that we currently have on the TeV map. So here are two of them, three

of them

VERITAS, MAGIC, and HESS, in Arizona, La Palma, and Namibia, respectively. That produces very nice pictures. They have an excellent sensitivity. They have a typical resolution of 0.1 degrees, and they produce very detailed pictures of individual sources. You can see one of them over there. However, they have two fundamental disadvantages. They are pointed instruments, so they have a very narrow field of view. A theorist tells you you need to look at that source as probably a TeV emitter, and they point towards that source and they say after a couple of hours yes or no. They cannot look at the entire sky. They can only look at a small part of the sky and detect a source there or not. So they're pointed instruments observing one object at a time. Also, this Cherenkov light is only visible in dark nights and when the moon is away from the source. So, they have a very limited duty cycle. And remember that I said these gamma ray emitters flare occasionally. Well, you would never know with this instrument because you're only looking at one particular source. If that source flares, you will catch it, but you won't catch what's going on anywhere else in the sky. So that's where HAWC comes in. The water Cherenkov detectors work differently. They measure the particles that actually hit the ground. So, again, water is the cheapest detector material. So the particles of this air show are raining down onto the ground, move through a big water wall, you may generate Cherenkov light in water now with a 41-degree angle, and that is captured by photo multipliers. The instrument to do that was Milagro. That was an experiment in the Jemez Mountains near Los Alamos, and I only mention it because I was a postdoc on that experiment so I have fond memories of it. This was sort of the predecessor of HAWC. As a matter of fact, the photo tubes that we used in Milagro are also the same photo tubes to be used in HAWC. So this was a water filled pond with a light tight cover on top of it and operated between 2000 and 2008. This type of instrument has also pros and cons. The pros is it has a large -- and sees the entire overhead sky all the time. So it has a large field of view and a large effective area. The downside is the sensitivity is much lower, and the angle of resolution is much worse. So it's not 0.1 degree; it's only one degree or so. And the sensitivity is such that you can see over there a picture that this instrument made of the Crab Nebula, but to make this kind of picture, it took eight years of data taking. So, this is what one of these Cherenkov telescopes can do in a matter of an hour. So, in order to improve on Milagro, what we thought is, well, height matters. So you can see over here, again, this cascade on the right-hand side, you see a plot that shows the number of particles as a function of altitude. So you see this little bulge where it reaches the shower maximum, and then the shower dies out. So we moved the experiment up from Milagro altitudes over here to HAWC altitudes, and we get a lot more particles. So that's where this picture comes in. it's a high altitude, 4,100 meters compared to the 2,000 something that Milagro was on. You can see, by the way, the lava stream here very nicely. It does look threatening.

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So this is, and the idea then is, of course, that you have the cosmic ray air shower come down over here, and then you catch it with your instrument.

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This is Brenda Dingus. She used to be a physics professor here. She's now at Los Alamos National Lab, and she's the spokesperson, she was the spokesperson of the HAWC experiment. So, where is it? That's where HAWC is situated. It's in the Parque Nacional Pico de Orizaba. Pico de Orizaba is that mountain. It's about two hours or so from Puebla, which itself is one half hours from Mexico City. So it's right there. The HAWC is at 4,100 meters. Pico de Orizaba is the big one, the volcano. Sierra Negra is also a volcano. And there's the large millimeter telescope is already sitting there. So there was already before we came some infrastructure. This is the HAWC site as seen from above. This is the direction to Vera Cruz. This is the direction to Puebla and Mexico City, and this is, I think, from Google Earth. So it's about 160 by 160 meters. That's where HAWC sits. So, what is it, then? Well, just as a fact sheet, it's 22,000 square meters of tanks with a 57% coverage. Of course, there are gaps between the tanks. It's made out of 300 water tanks, and they are 7.3 meters in diameter and 4.5 meters in depth. Each of them has three recycled Milagro photo multipliers and one new one that has a higher quantum efficiency at the bottom of the tank. So the idea, again, is you have this tank, a relativistic particle from this air shower hits the ground and goes through your detector, so it produces Cherenkov light that's recorded by the four PMTs, and the timing and the charge that each photo multiplier sees helps our to reconstruct the direction of the primary particle to about 0.5 degrees, which, for an astronomer, is shocking but for this type of astronomy it's actually fairly good. This is an event that's played. The color, so the size of the blobs tells you how much charge was deposited in the tanks, and the color tells you the timing. So blue is early, red is late, and you can see that the shower obviously came in from the right-hand side. So you can reconstruct the shower just by the timing of the tubes, the different timing of the photo multiplier tubes. So, inside the tank you have a light tight bladder which is blown up here with just air in it. It's dark from the inside. And the tanks are not made out of plastic like -- tanks, for example. They are made out of steel so they can be constructed at the site because it's rather remote. You can't bring these huge tanks up. By the way, Milagro, as I said before, was a huge soccer field size pond. This is a national park so it's not possible to just dig a pond there. We had to build these water tanks and put them right next to each other so that it looks like a pond. This is how they are built. It's kind of a cute sort of one picture per couple of minutes. So you built a little trench there, and you put in the first one. Put the roof on, and rather than building up, this is lifted and then you add one every time you lift a little bit. And there it is. It takes about a day, not 10 seconds. So, the trench is filled with cables. One of the first interesting number about HAWC is it's about a cable length total of about 180 kilometers. I have another fun number here. My favorite picture again. So we have 300 water Cherenkov detectors with 185,000 liters of water each. That makes 55 million liters of water. The population of Mexico is 120 million people. That makes half a liter of water for every woman, man, and child in Mexico. So, and this is the building of the experiment that we started somewhere in 2010. Then in 2012, this side was leveled, and then the first detectors were built August 2012, and the last picture over there is late in 2014. This is taken from the mountain. It's one picture per day. So you can see how it was built. At this point, we started data taking. So with the first 30 tanks, we have taken a little bit of data. This is 111 tanks. So we took data with that for a while, and this is about half the tanks. In the middle you can see the counting house. That's where the electronics come together, the cables all come together and the electronic sits in the middle to keep our cable lengths short. There we go. So, inauguration day was March 20, 2015, as I said. That was last Friday. In the middle is France Cordova, that's the director of NSF, and she has a little red button there that she presses. And that is the official start of full operation of HAWC. Okay, so this is the HAWC operation, a list of the institutes. It's sort of half Mexican, half USA. You can see that the biggest institutions in the US are Maryland and, actually, Wisconsin. Los Alamos also. This is the HAWC operation at one of our last inauguration meetings in October 2014. It's a collaboration of about a hundred people or so. So it's not a small experiment, but it's certainly much smaller than modern particle physics experiments. This is the local group. So, on the top row are our four students, and in the bottom row, post doc and research scientist. So, one thing that I want to talk about before I go into the results of HAWC is HAWC, of course, also see cosmic rays. As I said before, cosmic rays at these energies are scrambled so they don't point back to their sources. The problem with cosmic rays is that they're about a thousand times more abundant than the gamma rays we are looking for. So you have to find the gamma rays in a background of cosmic rays. And the way that HAWC does that is the following one. You can see a cosmic ray induced shower on the left-hand side and the gamma ray shower in the right-hand side. And you can see that the gamma ray shower, because it's purely electromagnetic, there are no high energy particles like pions produced, is very, very smooth. So you have the core where most of the energy lands, and then it sort of goes off at further distances. For the proton shower, you have, again, the core here in the middle, but then if you go away from the core, for example here and here, you have, or for example here, you have tanks with very high charges far away from the core, and that's indicative of -- proton showers. You don't see that in a gamma shower. So what we count on is if there's a large signal very far away from the shower core, that event we throw out because it's likely a cosmic ray. So we look for spotty events and throw them out and keep the smooth ones which are purely electromagnetic and these are gamma ray induced events. You don't have read this. This is just a list of the science goals. The important one is we want to provide an unbiased survey of the northern sky at TeV energies. The map that I showed you with 160 sources as produced by instruments that look at particular sources only. They don't have a view of the entire sky. And that's where HAWC comes in. So we want to make a picture in TeV gamma rays of the entire sky to discover sources that these instruments have not discovered. Of course, we work together with them now. If we discover a source of TeV gamma rays, we will ask those instruments, which are more sensitive if they know where too look, we ask them to look at that particular spot and see what they see there. So this is really a complementary experiment to those air Cherenkov telescopes that look at particular spots on the sky. So, we are looking for the galactic sources of gamma rays, we are looking for extragalactic sources of gamma rays, we look for Asian flaring activity. This, again, is an instrument that sees the entire northern hemisphere all the time, so if there's a flare going off somewhere in one of these extragalactic objects, we can alert the other instruments to go to that spot and watch that object. So, okay, let me talk about the first results from HAWC now. As I said, we haven't been data taking only since last Friday. We have been taking data for almost two years now with smaller parts of the detector. But the smaller parts were already more sensitive than Milagro was over those eight years. And I will talk a little bit about the results here. The first one is a little bit of a peculiar result, and that's the shadow of the moon. So, if you think about the cosmic ray flux coming in from all directions, pretty close to Earth there's a large chunk of material that blocks them, and that's the moon. So cosmic rays that come from the direction of the moon get stuck behind the moon, and we don't see them. So a nice thing about the moon then is it sort of serves as the first thing you would see in this instrument is the shadow of the moon. And it tells you, if you see it, that all your calculations of directions and so on are probably correct. So, that's the first thing that when these instruments get turned on they want to see. So the moon's apparent diameter is about 0.5 degrees, and the width of the moon's shadow is a good way to test the angle of resolution of the experiment and the pointing accuracy of the instrument. This is the moon in HAWC. So you notice that it's not centered at 0-0. It's not centered at the middle of the plot. So it's slightly shifted. It's shifted by a little over a degree in one direction. And the reason for that is that cosmic rays, again, are charged particles, and one of the magnetic fields they have to go through is the Earth's magnetic field. So they get deflected by exactly that amount in the Earth's magnetic field. And that's actually also a good thing because it tells you, gives you sort of an energy calibration of your instrument. It tells you what the main energy of your particles, your cosmic rays is from the deflection of the moon's shadow. So, when my postdoc talked about this at the conference, it was actually picked up by BBC and then by an Australian radio show which titled it "HAWC Discovers Moon."

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But it's kind of an independent detection of the moon. If you didn't believe it was there, it is, it certainly is. So, what about gamma rays? We have the first gamma ray map from HAWC 111. So that's the 111 tank version of HAWC, 206 days. And, again, you can start to see the galactic plane in the center over there. These gray spots are spots that we can't see. They're in the southern hemisphere. So HAWC is at 19 degrees latitude. It sees the northern sky. It does not see through the Earth in the southern sky. It's a good reason to build a southern hemisphere HAWC, which we will try to do after we have got this running. So, remember, this is only a third of the detector, and we already see a couple of sources there. Well, actually, I can point them out here. It's the galactic plane. It's a blow up of the galactic plane. Markarian 421 is one of those AGN, the supermassive black holes with the jets. We see that one over there. And then the Crab Nebula, the standard candle of TeV astronomy that we see with very high significance already. So, this is the same map. This time at equatorial coordinates. So this time it's not the galactic plane that's in the center here; it's the Earth's equatorial plane. So the dotted lines over here in this plot mark the galactic plane. So we can see again the flux from the galactic plane sitting over here. The galactic center is now in this plot is over here. The Crab Nebula is over here, and this one over here is Markarian 421, which was kind of at the edge of the other plot In this plot it's right at the center. So these are the two prominent sources that you see. And the reason why I bring this different coordinate system up is that we have a very nice sort of comparison to the Fermi satellite at GeV energies. What you will see in the next plot, this is again the HAWC map, it will go into the Fermi map now. So, Fermi, of course, it's a satellite so it has a full view of the sky. But you can see that we're starting to see the same thing. You can see the prominent source is Crab and Markarian 421 and the galactic plane going back there. So, remember, this is only a third of the detector for 200 days. So with a full detector, we'll fairly soon get to a picture as exciting as Fermi's. So another thing, we can, of course, look for the time dependence of these sources. This is the time dependence of Markarian 421 plotted here. And then you see sort of video of that. And there it flares. And flares a little bit more and then goes away. So most of the time we are not sensitive enough that we can see it continuously, but in a flaring state, we can already detect it with a detector that's only a third of the size. There it goes again, right? And that's this flaring state over here that happened. This is days after we started data taking, so it happened somewhere in 2013. There it is again. So we can do that, of course, now for every spot on the sky. So, one last thing, and that is the cosmic ray sky. I said before that cosmic rays are particles that are charged and therefore are deflected in magnetic fields. TeV energies are completely scrambled in magnetic fields. So the arrival direction of cosmic rays should be completely isotropic. That was what people thought before people in Milagro actually started to plot the background. So you can take these cosmic ray particles, of course, and also make a sky map of these particles. And it should be completely isotropic, but it was not. As you can see over here, this is the HAWC sky map for a couple of months of data taking. It is most definitely not isotropic. There are regions on the sky with an enhanced flux of cosmic rays. There's a region A, which is actually the sort of hottest thing HAWC sees on the sky, it's not a gamma ray source. It's a source of excess cosmic ray flux. And we have no idea why this happens and where these hotspots in the cosmic ray sky come from. They have been detected by the instruments now. There's a similar instrument in Tibet that's kind of similar to Milagro that's also seen these hotspots, and the IceCube instrument, which is also sensitive to cosmic rays, has made a cosmic ray map of the southern hemisphere and has seen similar hotspots. So, it's completely unclear where these hotspots come from. They are very visible on the HAWC map. It could be that these cosmic rays are sort of funneled in certain directions by magnetic fields that are nearby. That's one theory. It could be that they point back to some nearby sources. Although, the radius of these sources of magnetic fields is so small that there's no way that they can really point back to their sources. So it's a complete mystery. There was a first paper published in 2014 that shows these three regions of enhanced cosmic ray flux. And it's an access of about one per mill excess. So, as you can see over there, we see a statistical significance that's over 20 seconds. It's really prominent on the sky, and it's completely unknown where that comes from. So that's another sort of mystery to solve with HAWC. So let me summarize then. Again, the picture of HAWC. We talked about TeV gamma ray astronomy being the energy frontier in astronomy HAWC is designed to perform a wide field of view synoptic survey of the TeV sky. We are a large field of view, the entire northern hemisphere, the northern sky. We do an unbiased survey in TeV energies of the sky. We have a large up time. We want to look for transient sources and alert other instruments in all other wavelengths that there is something going on at a certain spot in the sky so that they can look there. And we have hopefully a chance to identify cosmic ray sources. So, in summary, it's an exciting time, like every time that a new window in the universe is opening. Let me stop with a sort of Wisconsinite picture of HAWC.

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And these web pages. Thanks a lot.

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