 a lot of good stuff here and so let's go ahead and get started here. So hello everyone and welcome to the November NASA Night Sky Network member webinar. We're hosting tonight's webinar from the Astronomical Society of the Pacific in San Francisco, California. We're very excited to welcome our guest speaker Dr. Francis Halzen from the University of Wisconsin at Madison. Welcome to everyone on the YouTube live stream and we're very happy to have you with us. These webinars are monthly events for members of the Night Sky Network. For more information about the NASA Night Sky Network and the Astronomical Society of the Pacific, check the links in the chat. And before we introduce Dr. Halzen, here's Dave with just a couple of announcements. Alrighty, hi everyone. I just will try to keep this brief but I've got a few good messages for y'all. Let's see. First of all, just want to remind everyone to get ready for the PIN program. The PIN shipping department is going to be on the road for the next week and a half. So if you haven't ordered PINs today, they will get to you. It just might be about a week and a half early December before they get to you. That is me. And remember also if you want the PINs for your outreach programs to add reports to your events for this year to qualify. We don't just need the events on the calendar. We also want to find out how the events went. So we just need information on just a few of those events. And here we get to go. And our PIN information page is on the Night Sky Network page and in your newsletter and I put it in the chat. Also, once again, the NASA Eclipse Ambassadors off the path program is recruiting astronomers and you'll be paired with undergrads to bring the science of eclipses and eclipse viewing tips to communities that aren't going to be on the path of totality in 2024. And don't worry, you can still travel to the path. We just want to help, we want you to help get your communities prepared for it. So if you need to, if you're interested and would like to apply or no more information, go to eclipse ambassadors.org and also the link is in the chat. And just kind of been on a side about that. We have lots of undergrads who have applied and we right now we need amateur astronomers to apply. So, you know, right now we've got an awful lot of eager undergraduates who we're not going to go to partner unless you all apply. Don't let those undergrads go lonely. Please support an undergrad and apply for the Eclipse Ambassadors program. Cue the Sarah McLaughlin news. Okay. Oh yeah, another fun tip. The ASP's annual meeting is back and it's virtual again this year and it runs from December 8th through the 10th. The focus this year is space telescopes and bringing their science down to earth and astronomers, professional amateur, unclassifiable and informal educators are all invited to join this community and really science communicators of all stripes. Go on down to the ASP annual meeting. You can find details at the ASP's website at astrossociety.org and the link will also be in the chat. And one last final one. If anyone's paying attention to any NASA news today, Artemis 1 will be launching around I believe 1 0 4 a.m. Eastern time tomorrow slash late tonight and you can watch all the coverage on NASA's YouTube channel as well as on NASA TV and it's going to be on about probably about half an hour after we end tonight. So you have enough time to get some snacks for your Artemis launch watching. So enjoy and that is it for me. Take it away, right? All right. Thanks, Dave. For everyone on Zoom, you can find the chat window and the Q&A window down at the bottom edge of your Zoom window on your desktop. Please feel free to greet each other in the chat or to let us know if you're having any technical difficulties. You can also send us an email at nightskyinfo at astrossociety.org. Please put all of your questions in the Q&A window. That really helps us keep track of things. If you put them in the chat, chances are that we'll lose track of them. So put those in the Q&A window and we just hang on for just a moment here. All right. Welcome again to the November webinar of the NASA Night Sky Network. This month, we welcome Dr. Francis Halzen to our webinar. Francis Halzen is a mispronounced Vias and Gregory Bright Distinguished Professor of Physics at the University of Wisconsin Madison. He's the principal investigator of IceCube, a cubic kilometer neutrino telescope buried in the Antarctic ice at the South Pole. IceCube's first observations of high-energy cosmic neutrinos garnered the 2013 Physics World Breakthrough of the Year Award. In September 2017, IceCube detected a high-energy neutrino from the direction of the blaze hour called TXS 0506 plus 056. This was the first-ever evidence of a source of high-energy cosmic rays whose origins have been notoriously difficult to pinpoint since they were discovered over 100 years ago. Dr. Halzen is the co-author of Quirks and Leptons, a classic textbook on modern particle physics that continues to be used extensively throughout college campuses today. He's written a large number of publications and has written or edited several books. His essay, Antarctic Dreams, about the early days of Amanda, IceCube's precursor, was featured in the Best American Science Writing for 2000. Please welcome Dr. Francis Halzen. Good evening. It's a pleasure to be here. Thank you for this nice kind introduction. So I'll try to share my screen. And now I think we should be all set. Yep, that looks great. Okay, looks great here too. The topic of this talk is the Under the Ice Neutrino Observatory in Antarctica. And so I will try to explain in less than an hour a lot of things. So this is the menu for today. And as you can see, I have to introduce what a neutrino is. In case some people don't know, then I have to explain what neutrino astronomy is. Then I will explain that we actually built the first neutrino telescope, which is called IceCube. Then I'll show you the first neutrino maps in of the sky, which were rather surprising to us and to everybody else, I think. And then I'll show you the first neutrino stars. So let's start because this is a long menu. What's a neutrino? That's easy to explain. You learned in high school that the world is made of protons and neutrons that make nuclei. And then you put electrons around it. And then you get atoms. And that's what we're all made of and the universe made of. That's actually not quite true because in Cambridge in the 1930s, they noticed that a neutron changed into a proton occasionally. And when that happened, the neutron changed into a proton and electron. And you know from the simple rules of billiard ball physics, if the proton goes in one direction, the electron goes in the other direction. Occasionally, they saw in their experiment that this was not true. And so the electron and the proton would go in the same direction, will violate energy and momentum conservation. So Pauli in his wisdom said, well, we have to save the laws of physics. So there must be another particle, which was called a neutrino eventually, that takes off in the other direction. And that restores Newtonian mechanics. And so the idea was they couldn't see this particle. It was just kind of an agent that made the possibility of changing neutrons into protons, which by the way is nuclear physics. And so that's where the name ghost particle come from. It just was there to say physics from itself. But you couldn't really detect it. So he told the fact that it would never be detected, which was really not true. In fact, Alice and Mott reconstructed the distribution, the mass, which was close to zero of this particle. And basically, by today's standards, they detected the neutrino, but they never claimed detecting a particle, they couldn't imagine that the world had more than three particles. So, but you've got the idea to build the universe, you need neutrinos. And in fact, of these four particles, neutrino is the most common one in the universe. And without it, there is no nuclear physics. And without nuclear physics, we wouldn't have had a big bang. Stars wouldn't explode. The sun wouldn't shine. You emit neutrinos, we produce neutrinos at Fermilab and at Sun. So neutrinos are everywhere where there's nuclear physics. And look at this picture on the top right. This is very important picture. You see, this is a nuclear reactor. It's covered with water to shield yourself from the charged particles that are emanating from the reactor. And you see that water is blue. So charged particles moving through water emit blue light, which is called Scherenkov radiation, but that's not important. The blue light allows you to see the particles. I'll come back to that later. And there is another important and unfortunate place where you produce neutrinos. That's about 20 kilometers above our heads in the atmosphere. Cosmic rays come into the atmosphere. They eventually meet a neutron or oxygen nucleus. They make a nuclear reaction. And the nuclear reaction makes pions that decay neutrinos. The details of these are not important. But what is important is that we are bombarded all the time with neutrinos reaching us from the top of the atmosphere. And so imagine you want to look at neutrinos from space or from the universe, like you do with light. Then you look at these clouds of neutrinos and they never go away. So this is part of the story that I will have to explain to you. But so the conclusion is, if you detect neutrinos, you also detect cosmic rays because you need cosmic rays, which are protons mostly to make pions that decay in neutrinos. So if you find neutrinos in the universe, you will find where cosmic rays are accelerated. And so you are solving the cosmic ray problem, which is the oldest problem in astronomy. Cosmic rays were discovered by Victor Hess in 1912, more than 110 years ago. We have no idea where they come from or how they accelerate. But as you will hear at the end of this talk, that is changing. So what's neutrino astronomy? This is the sky when you look at it with your eyes. I will always show the sky in galactic coordinates. So you see in the middle of the ellipse, you see the galactic plane. And so I am a particle physicist, I think in terms of photons. And so these are photons for one electron volt. If you look at the sky in photons of a 10,000 of an electron volt, then the sky looks like this. That's the microwave background. And what you are looking at is 411 microwave photons per cubic centimeter everywhere in the universe. So if I move to the other side of the visible spectrum, this is the gamma ray sky. Now the energy of the photon is one giga electron volt. And more. And so you see the sky looks again different. And so astronomers have looked at the sky as you know very well in all these colors of light from radio to tiny wavelengths of high energy photons. But this game stops if I go farther into the blue to wavelengths of 10 to the minus 21 meter, the sky turns dark. There is no astronomy. We don't see the universe at these small wavelengths anymore. The energy of the photons is a thousand TV. By the way, TV, I live here 200 kilometers north of Fermilab. Fermilab is an accelerator that makes one TV accelerates one TV protons. And at some they make beams that accelerate 14 TV protons. So this I will use this accelerator unit as my unit in this stock. So this is another way of looking what I just told you. You see, look at the top. There are the radio waves of course that will play a role in this stock later. I go microwave, optical, x-ray, gamma rays. And then suddenly this game stops. The universe turns dark and we have never seen the universe beyond this demarcation line. In fact, what this line shows is if you reach energies of about a thousand TV, then you can barely see the galactic center. And the reason is shown in the next slide. Say you have a galaxy far away just to take an example that emits a photon of these energies. Well, it will never make it to your telescope on Earth because that photon will interact with one of these 411 microwave photons per cubic centimeter and makes an electron positron pair. What's wrong with that? Well, now you have charged particles with charged particles. You cannot do astronomy. And we know this very well because the cosmic rays themselves are charged and their path is bent by the magnetic fields in the universe by the magnetic field of inside our own galaxy. And so they don't tell us where they come from. That's why we have never solved the mystery of their origin. So neutrinos don't have that problem. They don't interact with anything. They are ghost particles, remember? They reach us from the beginning of time from the edge of the universe. And so they can help us to look at the sky in the dark region of the universe. And so is this an interesting problem? And I'm a particle physicist, and so I'll give you my own answer. I remember when in 1991 a modest detector in Utah, the flies eye detector, observed the cosmic ray of 300 million TV. This is 100 million times the energy of the highest accelerator being produced at CERN and Geneva. We are looking here at particles with a wavelength of 10 to the minus 16 times the size of an atom. And so if you used state-of-the-art magnets that are used at CERN and Geneva, you would have to fill the orbit of Mercury to produce that particle. And how can you resist not to spend the rest of your life trying to solve this problem? So all we have to do is detect these neutrinos. And so that's the task we set out. How to build neutrino detectors? We know. You need water and you need light sensors. You see the photomultipliers on the wall on the wall of this tank. And so the water is rising and turning this into a neutrino detector. The problem with this beautiful detector in Japan is that the theorists in their wisdom told us that this experiment is 10,000 times too small to detect neutrinos from the cosmos. And even if you built one 10,000 times bigger, you are not guaranteed seeing anything. But that didn't face us. And this is Moshe Markov on the left. And he had already told us at a talk he gave at a meeting in Rochester, New York in 1960, how to solve this problem. What you do is you put a light sensor deep in the ocean or in a lake. And you look for particles coming through the earth. If you see particles coming through your feet, it can only be neutrinos. Nothing else come through the earth. If it's a muon neutrino, well, what do these particles do? They fly through your detector. You don't notice them. Neutrinos have no charge, electric charge. But occasionally, in our case, about one in a million times, they will crash in a nucleus in the ice, in the water, so real the ice, and make charge particles. You get a nuclear reaction. They make charge particles. And if this is a muon neutrino, they muon neutrinos produce a muon. And that muon will fly at a speed of light through your detector. It's charged. It makes blue light because the speed of light in water is three quarters of the speed of light that that muon is going. It's going at almost the speed of light. It makes a shockwave. It's like a boat outracing the waves in the water. And the shockwaves allows you to tell you where the muon and the neutrino came from. And so you have a telescope. So you get a flash of light that lasts about a million of a second. You reconstruct that flash of light and you detected a neutrino and you know where it came from. So I think I explained all this. So here is another picture. Here you see each of these dots is a photomultiplier. It has the size of a basketball. And when it takes light it lights up. This is the online display of the ice cube experiment. And you see the neutrino came in. It crashed into a nucleus. It made this muon and this muon travels at a speed of close to the speed of light through your detector. It deposits 2600 TeV inside the detector. And that's interesting because with that much energy it's very unlikely to be produced in the atmosphere. If you didn't get a picture yet, I show you the movie. So you see the muon comes through the earth 11 degrees below the horizon and flies close to the speed of light to the detector depositing 2600 TeV of energy. So Markov actually suggested to implement this idea in the water. And there was a heroic experiment a long time ago in Hawaii that unfortunately failed. And so we had the idea that it actually may be simpler to freeze photomultipliers in ice than to put them in deep ocean waters. This may be counterintuitive but history has proven us right. So where do you find ice to deploy such a detector adept at a south pole? So you build a detector or its component at least in Madison, Wisconsin in the physical science laboratory. The equipment goes to Christchurch, New Zealand at the airport. It goes to McMurdo base and from there another three hour flight, nine hour flight to McMurdo, three hour flight to the south pole. It can also go by ship from Port Unimic close to Los Angeles directly to Christchurch or McMurdo. So this is the south pole. That's what you see when you're at the south pole. You stand here on three kilometers of ice which is what we need for this experiment. And if you look in the other direction you see a station which is another thing we need. There is ice everywhere but you need a lot of ice in a place where you have an infrastructure that allows you to build a particle physics experiment. And the other thing is this is the station at night so you can actually stay with the equipment 24 hours. So the south pole was like a small miracle that happened to us that made this experiment possible. There was another thing that made this experiment possible. I cannot show you the glacier at the south pole but imagine this is a three kilometer ice sheet. We found that if you go one point five kilometers deep, slightly less actually, you find the clearest solid that nature ever built. You cannot even build a solid like that in the lab. The blue light I've been talking about can travel in that ice over distances of 100 to 300 meters. And so that's what you need to build a particle physics detector. Why was this ice so close? It was a serendipitous discovery but it makes sense because you are looking at the center of the detector of ice that fell on Antarctica 50,000 years ago. It's ultra pure and compacted into the ice that's in the detector. So this is a picture of the south pole. This is the south pole station. The geographic south pole is here and this is the ice cube construction project and everything comes in by C-130 planes. This adventure started very modestly. In fact, I was a theorist. I didn't have a lab so I removed the ping-pong table, the table tennis table. We played table tennis on and turned it into a lab where we put basketball-sized photo multipliers in pressure vessels and then hope to deploy them at the pole. These were three graduate students that were once theorists. This is a respectable physicist who now lives retired in Hawaii, Bob Morse, and this is Seraptila who is at the University of Delaware. So this is a picture of a 10-inch PMT. You buy it in Japan. That's easy. And then you melt it in the ice. That's not so easy. That's the second problem we had to solve. I'll come back to that. But here you see a picture of the south pole in 2000. And by then we had deployed some 600 of these photo multipliers and we observed events like this. And you see this is a mule on track going through the earth, lighting up these photo multipliers tubes. And at that point, I must say in the whole adventure, this was the most exciting thing, the most exciting moment that we actually knew this was going to work. And I should point out that this was not always that easy. You're listening to a theorist, right? What do you do? You publish in Nature, claim you have solved the problem on how to discover the cosmic rays. You get help from the economists who they were intrigued by this experiment and provided our Amanda logo. And so then you say, now I'm going to finally build a kilometer cube detector. And so it becomes an industrial type project, five more than 5000 photo multipliers. If you could go in the detector that we built, it would look like this. You would see this clear ice. You would see strings with light sensors, a kilometer long, one every 17 meters, you go 125 meters away, you see another string, 86 of these strings fill a kilometer cube of ice, bubble free ice below 1450 meters, ultra clear with 5,160 of these light sensors. So to avoid all questions on this, how do you melt these things in the ice? You never drill holes. You change ice into water, sink the equipment in and let it freeze. It takes a 4.8 megawatt heating plant and it delivers 200 gallons per minute of boiling water through the nozzle of this equipment. So the first 90 meters are snow and you just melt it. So you melt big enough hole for the equipment to go in. Then comes the hot water drill, which is basically a nozzle that puts out this hot water under pressure and it falls by gravity. And two days later, it's at about two and a half kilometers. And then you pull the drill out and then you are ready to deploy your equipment. You realize ice is an insulator so that water stays liquid for quite some time. So you have the time to deploy your equipment. So this is the hot water drill. It's like a circus train, but except on wheels it's on sleds. This is the hose that goes to a depth of two and a half kilometers in one shot. Two days you are at depth. This is the power plant 4.8 megawatts, 40 car wash heaters that are driven by generators that use normal fuel that comes in by C-130 plane at the time. So when this operation ends you see the drill comes out, the hot water drill comes out, and you have now your column of ice transformed into water. So at this point you move on 125 meters further, but here are 60 of your light sensors in pressure vessels with electronics that I will not go into, but you are ready to deploy them. So what you do is you attach them every 17 meters to a cable that brings down the power, but also brings up the information that your sensors detect. So all the light detected by the sensors is converted to a digital signal which goes up to the surface on the same cable that brings the high voltage. So you can do this for about two months and you see all these cables, 20 cables here from strings deployed in September, in December and January of that year, and they go into this two-story building that's filled with computers that reconstruct these light signals in the way I explained before. So here you can see ice cube in action. When you turn on ice cube what do you see? You can guess, you see these neutrinos and not just neutrinos, also muons raining down on your detector from the atmosphere. But remember that background is not there when you look up through your feet, and so here you see the detector reconstructing muon tracks. Sometimes these muons come in bundles. You see this bundle of cosmic rain muons flying through the detector, but we are interested in the tracks coming up and trying to eliminate this mess. You should realize that you are looking here at 10 milliseconds of data. So what that means is we detect 100 billion cosmic rain muons per year, 100,000 atmospheric neutrinos, and we are looking for some small signal of high energy neutrinos reaching us from the cosmos. So in a background of 3,000 muons every second and one atmospheric neutrino every five minutes we look at this cosmic neutrino. When around the time we had built this detector actually the new scientists gave us a score of six to one. They had a page you could bet and you see there you could bet on big science project and they gave us six to one for discovering cosmic neutrinos and I can tell you I didn't sleep well after seeing that. In fact they also gave six to one for soon discovering the Higgs boson and if you look here this is LIGO and they gave LIGO 500 to one for discovering gravitational waves. So you see the luck we had in the last decade all these despite their odds all these discoveries became reality. I never really worried once we build it what we would see because you know in your elementary astronomy text right you've seen these tables that tells you why people starting with Galileo build telescopes and what they eventually do with it and the moral of this table is that you always do something interesting in fact more interesting that one you could imagine and so you know when we build it and it worked I was never worried we'll discover something. So we started taking data almost then no more than 10 years ago now and so we were actually looking for these protons but in a very original way I told you how the universe turns dark because the photons are stopped by the microwave background the highest energy cosmic rays they are also stopped by the microwave background they don't reach us so they find a microwave photon make a pion that makes a neutrino and these neutrinos have enormous energies because only the highest energy cosmic rays can do this like the particle in Utah and it would fill your detector with light and even a theorist can analyze the data and we analyzed two years of data and we found nothing even though you can calculate this and it's interesting why we didn't find nothing but that's a different talk but while looking at these two years of data we found this event in fact we found two of them and you see there is just serendipitously and you see there's no muon track that is because this is not a muon neutrino this is an electron or a town neutrino in fact we now believe this one is a town neutrino and so what happens is the neutrino interacts it makes charged particles no muon track but these charged particles make blue light and the blue light is like a bomb of blue light that explodes in your detector and in fact from the size of this bomb you can tell what the energy is and the energy is a thousand tv and we know it very well because we see all of it in this case and it's unlikely that the atmosphere produces a neutrino of this energy and in fact we found two we didn't claim anything but then we went through the two years of data specifically looking for events like this and they are not that difficult to find this is an event superimposed on the data center in madison where i'm sitting right now i sit here somewhere this is the lake and the madison campus and the event you just saw that's its size is that the size of like five city blocks and there are 300 sensors that report detailed information on 300 000 photons that we detect and so we looked through the two years for more events and we found 26 more and so then we declared victory and published in science that we had discovered cosmic neutrinos and the next week we were declared the breakthrough of the year which made us very very nervous but eventually not only was this a discovery it we confirmed in a totally different way this discovery ourselves you realize we were supposed according to markov to look for neutrinos coming through the earth so coming from the northern hemisphere and look through our feet at the south pole and we did that as well and we discovered cosmic neutrinos a second time in fact we have by now discovered them in four different ways i won't go into but here you see the num just this is the plot that just shows the number of muons we detect as a function of the energy of the neutrino and you see this is a hundred gv a tenth of a tv my unit of one fermilap accelerator and that's the threshold lower energy neutrinos in this analysis we don't see we can see them in other ways but so you see your detector becomes more efficient you see more and more neutrinos but these are neutrinos coming at the rate of one every five minutes from the atmosphere they're pretty useless then you see when you go up to higher energy the energy the atmosphere is not very good at producing higher energy neutrinos we understand the mechanism we can predict this flux actually and you see we predict and measure it it's the blue line but then if you reach about a hundred tv you see we see more neutrinos than are produced in the atmosphere and of course they come from the cosmos they are at they come at a rate of more than 200 per year and they are in this tail so with the Markov method we confirmed our previous measurement so what do we learn from these measurements first of all let me show you this is a map i cannot show you all our data this is one year of data that's the resolution of power point there are 138 000 neutrinos in here and 200 slightly more more like 220 come from the cosmos the purity of this sample is 97 percent so we beat this background we're saving a pure sample of neutrinos with only 3 percent background left in which leave these 200 cosmic neutrinos 12 of them are nicely separated about one a month nicely separated from the atmospheric background and so this happens for energies that are about 100 tv or more so let's just look at the highest energy neutrinos and what do you see we don't see the galactic plane now as you know very well we live in a galaxy and we are first supposed to see by simple geometry the sources in our own galaxy we don't we see the rest of the universe and that's consistent with this picture or you know that neutrinos come from the beginning of time and the edge of space and well there they are and they actually outshine our own galaxy uh i won't come back to this but you know we the galaxy is there at a level of 10 percent and i'm not allowed to tell you but you know we'll you will find out about it uh sometime when science allows me to tell you but uh i see mean by science magazine so no galactic plane the other surprise is this is the total light in the universe at different wavelengths and you see here this is the microwave background so this what astronomers call new f new that's the energy that's reaching us from the universe in terms of microwave photons it dominates the sky this is the radio background here you see you go through the different wavelengths of light the infrared uh uh ultraviolet the x-rays and then you see the gamma rays and then you see the universe down there so this is actually the remnant of this gamma ray flux this gamma ray flux lives in the universe but it gets shifted it doesn't make it with its full energy through the microwave background and so we observe it cut off and this is an early measurements of the neutrino flux and this was a huge surprise our neutrino flux we see is larger than the flux of light in the universe this was not expected at all cosmic rays and neutrinos were supposed to be some boutique science uh done by original people and not paid much attention to well they dominate the dark universe and we'll have to deal with them if we ever want to understand the highest radiation that reaches us from there this is of course a great opportunity to do astronomy so I told you all these things and uh so the real question now is where do they come from you realize we have discovered we measure about one a month that we can point back to their source we've been doing this for 10 years so they have a resolution an angular resolution of better than 0.5 degrees not great but guess what we didn't find the sources so what did we decide to do well we decided to throw our data overboard which is called multi-message of neutrino astronomy so here top left you see this picture so this is a flash of light made in a microsecond and all this information you see and don't see in the picture goes up to our computers about 30 seconds later the computers tell you what the direction of this neutrino is and then you send it over a radium satellite to every astronomer in the world by astronomical telegram who wants to pay attention soon after we started this we struck gold this is the telegram sent to the astronomical community uh this is the neutrino it has an energy of 290 tv remember the demarcation line where the neutrinos become interesting is about a hundred tv uh it comes from orion and we send the coordinates to the astronomical community and found out that this neutrino points to what the astronomers call the blazar the accessor 506 plus 0.56 one of thousand blazar it's nothing special about it except at the time it was flaring you know blazar changed their flux over all timescales and this one was flaring so that made the plot ticking but this observation can still be accidental it can be actually it has a probability of being accidental of one in a thousand times however later it was discovered by ground-based telescopes magic in the canary islands that it emitted hundred gv gamma rays and uh as it's four billion light years away that means it really uh produced tv gamma rays that trickled through the microwave background down to the telescope at a hundred gv in fact we had no idea whether any astronomer ever paid attention to these telegrams but it turns out that more than 20 telescopes were looking at this uh in the direction of this neutrino at some point after it was uh detected in fact an optical telescope was looking at this neutrino uh 73 seconds after we send out the information then it was seen by swift by fermi uh and here a set of telescopes were looking at it to try to identify the back the the host galaxy and to measure the distance to the galaxy which eventually happened uh months later when the higher energy radiation stopped blinding the optical telescopes so we published this uh multi messenger observation but if you notice there is a second paper following the multi messenger paper by ice cube and what we did is we went to look at the nine and a half years of data we had collected before this event now we had a place to look and uh what we saw was basically nothing except for this huge burst in 2014 look this is a burst i've been talking about it's a burst that's dominated by this one neutrino but in 2014 we saw this burst that dominates the emission of this galaxy over the last decade it uh the energy of this neutrinos is adding them it's at the adding the luminosity for this galaxy it we have enough events to reconstruct the spectrum which e to the minus 2.2 and it was not accompanied by gamma rays in this case in fact i have my doubts about this but i won't go into this it turns out this was the observation by the way there is the galaxy this is where these neutrinos uh reconstruct uh the galaxy of course has a super massive black hole and it has a jet but uh we learned later the hard way by not being able to model the jet that most likely the neutrinos are produced close to the super massive black hole now uh before finishing an update i have already introduced this map and um already told you how difficult it is to find sources of this map however if you look at the 10 years of data this map is not totally uniform anymore uh and it has a there is evidence more than three sigma actually that this map is not isotropic and it's actually due to four sources that stick out by the way what you're looking at is not a map of the neutrinos it's a map of the p-value the probability that in a place like this for instance there is a cluster of neutrinos a neutrino star with on average higher energy of the neutrinos so just the probability that there is a neutrino source anywhere in the sky and actually the highest probability is here and that's ngc 1068 a nearby active galaxy and we observe with that uh 2.9 sigma after you take into account the look elsewhere effect which we can actually calculate it's totally given there are no hidden trial this is a real probability so the question is are these inter in interesting fluctuation is this a fluctuation or is it uh you know is it a real source or just fluctuate at an interesting place so besides to uh to answer this question by after 10 years making a major overall improving the detector so we improved the detector geometry we improved its calibrations and we now calibrated each of the 5000 plus light sensors individually and reconstructed the data with the individual calibrations and as we have kept all our whole data we can of course everything we do we can can apply to the 10 years of data we have so we learned more about the optic of the ice we have a program going on that keeps studying the optics and improving our understanding and the optics of course is uh but the the medium the ice the optics of the ice is by the is the medium through which the photons we detect propagate the shrink of light and so the better we understand the ice the better we can uh reconstruct the shrink of cones and light flashes that I've shown you before so we improved the angle resolution it's now 0.3 degrees with no systematics to be included it's absolute 0.3 degrees you know I know it's not great but it's quite an achievement uh given what we expected uh and then because we modeled the point spread function of the telescope with gaussian approximation we were partially blind because we knew this was not a good approximation for instance if you have a neutrino pointing there in that direction the blue line was our point spread function around the direction of this neutrino where in actual uh data tells you it's this the simulation tells you so if you if you model this correctly and you have your point spread function correct you'll you improve the vision of your your instrument you're not no longer partially blind and so we implemented all these things in one shot and applied it to 10 years of data and this is what happened to ngc 1068 this was the observation before recalibration and reanalysis by the way I forgot to mention we made improvements in the energy and pointing resolution also by using uh modern uh machine learning techniques rather than making cuts on the data so by improving the calibration you see how the neutrino moved closer to the actual position of the source and how eventually the resolution became better and so we moved from 2.9 to 4.2 sigma and you see whereas in the early analysis we found uh we found occasionally better and worse evidence for the source in the new analysis every year we see exactly the same thing and so that builds up after two years to this uh result where we now point better we have 18 neutrinos 79 coming from this direction with higher energies on average so this is uh a solid and as I said is 4.2 sigma includes all the trials to do these measurements which we count in a blind analysis so this is a real probability so another way of looking at this result is to do something very simple this is the distribution of the neutrino directions around the source so this is the direction of NGC 1068 this is the background in orange of atmospheric neutrinos and you see this excess of 80 some 80 neutrinos in the direction of the source what you don't see in this plot is that this excess also on average these neutrinos have higher energy and the combination builds up to this 4.2 sigma so there is another way of looking at this result and that is to uh look at the flux of neutrinos you observe as a function of the angle the zenith angle so zero is the horizon and one is neutrinos straight through the earth from the north pole to the south pole and so there you see NGC 1068 and that's the sensitivity of our telescope and the sources we look at are mostly around the sensitivity of our telescope this is the five sigma discovery and you see these sources sticking out and it's because of these sources that I told you that the map is no longer isotropic look what the second source is it's the excess of five or six although we are not looking for spectacular neutrinos or flaring events even if you just look for the time independent emission from the sky you find this source again and so I don't think there's much doubt anymore that we have established the excess of five or six as well with this analysis uh you can do I won't go through the rest of this plot it makes different assumptions on the spectrum and then these sources can rise and and fall but the conclusion is that we besides the excess of five or six at NGC 1068 we also see NGC 4151 emerged from this table and I like this because you notice the NGC 1068 and 4151 were the two sources that were identified by seifer in 1943 which as what we now call seifer galaxies was active course that are opaque to gamma rays and and that have these broad broad lines so this is nasa's illustration of an active core at the center the obscure core at the center of the active galaxy and the physics makes sense you know somewhere between the accretion disk and the base of a weak jet and the black hole particles are accelerated but they are accelerated in this region that has a is high as is obscure as a high density of protons more than 10 to the 24 per centimeter square and from which gamma rays cannot escape and so that's probably the accelerator of the 18 neutrinos I just talked about now I'm going to conclude at this point and the question is have we solved the cosmic ray problem of course protons that are cosmic rays make an active course neutrinos so well I let you judge but it's too close to call victory what I want to emphasize actually and what I hope I have tried to convince you of is that with multi-messenger astronomy we have the tools to finally solve this problem so let me conclude at this point and thank you and I want to recognize that I didn't do this all by myself and so ice cube is a collaboration of about 350 physicists even some astronomers and so I want to leave you with this picture it are all these great young people that did the work that I've been talking about thank you very much all right well thank you very much that was really really interesting and it's remarkable how we could use these this detector and the ice and Antarctica to determine some of the phenomena that's out there in the universe so we have a few you know quite a few questions here and I'm you know apologize right away to everyone that we whose questions we don't get to so we've got time for a few though so we had a couple of questions having to do with the detectors and they're wondering whether or not they they suffer displacement or damage because of movement within the ice sheet and whether you have to replace them every once in a while well the last question is easy you cannot replace them they're frozen in the ice forever the other answer to the question is they move they're sitting in glacier that runs off the west Antarctic mountains mountain range and so the detector moves uh 11 meters 10 to 11 meters every year with the glacier but at the bottom the temperature is close to zero sea and uh so it floats on ice we don't detect any any relative movement of the detector of the detectors relative to each other so it's totally stable also we have lost you know a handful of the 5000 under 60 in the last 10 years you lose quite a few when they freeze into the ice because this is a violent process I won't explain why but you lose at the level of 1% but after that the detector is frozen in the ice the electronics is s-50c and nothing happens to it you have a totally stable detector some of the photomultipliers have been in there for more than 15 years now so here's an interesting question and so do you feel that there are considerably more cosmic neutrinos than the current sensitivity and density of your detectors is capable of detecting no we detect the diffuse flux we detect gives us a measure of the total energy or the total flux of neutrinos that reaches us from the cosmos and we now know 90% is extra galactic and the galactic plane is at the 10% level so we know how many neutrinos reach us we understand the sensitivity of our detector remember I didn't point this out but by measuring the cosmic wave flux you calibrate the cosmic rate the the atmospheric neutrino flux you calibrate your detector and so we know exactly we know very precisely what a sensitivity is we have seen event individual event where we can calibrate our detector at high energy so we understand its sensitivity we see how many neutrinos we detect so we can reconstruct the flux of the universe so if we want to to detect more than one good neutrino amount or more than the 200 plus per year in that map we have to build a bigger detector so when a couple of people asked if using solid state detectors photo detectors would be eventually better than they no of course you know everybody is moving to silicon PMTs and so in these experiments you need a superb energy resolution and superb timing resolution and photo multipliers are still it also there she yeah so that's uh you know this is an interesting one and so I'm just going to read it the way it is the mu muons are charged so they're generating their own Cherenkov radiation but the neutrinos are only detected by scattered electrons so how can you deduce a direction from the scattered particles when there are so few events no no so the neutrino if it's let's take the case of the muon neutrino because that's the one the question that is referring to as you saw in the picture the neutrino interacts the muon moves in the direction of the neutrino and then the Cherenkov flight tells us the direction of the muon so if you know the direction of the muon you know the direction of the neutrino now you may ask how do I know that well as you told in the introduction I wrote the textbook about it that's why I know it but there is something called the standard model of particle physics and all the physics by which the detector operates like how much light the muon emits how much light are photomultipliers detect how the alignment of the muon and the neutrino at high energy all that is given by the standard model of particle physics so we know the physics by which the detector works even though I have not gone into it in detail so I want to come back another question here and I think it's related to one that you just answered a moment ago so this person wondered that if the universe is filled with the neutrinos how come the telescope is only detecting 200 per year of the ones that you're interested in well uh first of all remember 200 is not a small number because for every one of those there are a million that we don't detect and fly to the detector unnoticed so then the other question is what's the large flux and what's not a large flux it's large in the sense well once we understand the sources I will be able to answer the let me let me assume course of active galaxies are the accelerators of the cosmic rays that make the neutrinos then I can answer the question because I know how many active galaxies there are I can integrate them I know how many they each emit I can integrate them over the history of the universe and then I can tell you the total flux in neutrinos in the universe we're not quite there yet but anyone can do that calculation actually I'm sure every theorist working on this topic has already done it but that's a flux is the flux large or not well it's large because it's higher than the flux that reaches off us from the universe in light as I showed on one of these slides okay I'm going to do two more questions here and again I apologize to those of you whose questions we're not getting to and so we have a question here is do we know why neutrinos exist are they needed for the universe to exist I think that one of your slides I think one of my slides made this clear right we wouldn't be here without neutrinos there would be no sunlight no big bang no supernova remnants from which the material some of the material in our body is made so neutrinos no nuclear physics no universe so basically they're essential for they're totally essential so that's why I said that the world is made of neutrons protons electrons and neutrinos there's no universe without neutrinos and in the by the way if you count them they're dominated by the ones made in the big bang there are more neutrinos in the universe than electrons protons and neutrons so that's another way to answer the question so these are not our neutrinos which are relatively rare in a high energy but the ones made in the universe that's the largest number of particles in the universe are these neutrinos made in the big bang which have an energy of about two Kelvin instead of three and I lied I've been asked two questions a follow-up to that one because if there's that many neutrinos is this does it have any implications for the identification of dark matter or you know the nature of dark energy is that have any relationship to that it may it may not as we as we don't know what dark matter is and what dark energy is anything is possible although we try to do some things that to address the question first of all we look for dark matter uh in ways that I cannot explain in a short time but we look actually for neutrinos high energy neutrinos not nuclear neutrinos coming out of the center of the sun and that's a very sensitive measurement of uh the concentration of dark matter in the sun but as I said that's a different question but we also look for instance when the neutrinos reaches from the universe we look in the direction of the galactic center and if there is a lot of dark matter there which we are rather sure there is right there is a concentration of dark matter at the galactic center so we are actually looking whether neutrinos that arrive from the direction of the galactic center are blocked by dark matter and we don't see any evidence and so we can put a limit on the cross section for dark matter to interact neutrinos to interact with dark matter interesting tiny cross section so we do all kinds of things related to dark matter uh but uh again this is a very long talk which is different uh you know what I talked about about one third of the people in ice cube are working on this all the others are doing something else they're probably 21 hour talks that uh I'm not about to give now okay uh last question for sure and uh I think the same what if there is a nuclear detonation around the earth someplace would the array detect that uh yes uh I it's not quite big enough uh you know we did this calculations many many years ago I think the you would need about ten ice cubes or more less than a hundred more than ten now we are thinking about next generation detector that's ten times the volume but uh you would uh and then you could uh detect a nuclear explosion anywhere on earth so uh it's something I think that's a bit uh a bit unrealistic but not impossible the neutrinos unlike any other unlike the light you detect from a nuclear explosion the neutrinos would instantly tell you what uh what the power of the bomb is it's the only way so it's an interesting measurement but yeah if someone wants to build a hundred ice cubes we'll be ready to take the data there you go all right well thank you so much this is really really interesting and and thank you for joining us and thank you everyone for tuning in so you'll be able to find this webinar along with many others on the night sky network website in the outreach resources section each webinars page also features additional resources and activities tonight's presentation will also be on the night sky network youtube channel and join us for our next webinar on Tuesday December 13th when Dr. Robert Nemeroff returns to share with us the best of the astronomy picture of the day for 2022 so keep looking up and we'll see you all of you next month and thank you