Studying Quasars with Neutrino Astronomy (script) (Patreon)

Neutrinos are one of the most bizarre of known particles. Black holes are probably the most bizarre of astrophysical objects. Makes sense we should use one to study the other, no? Well, today we’re doing just that.

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There’s been a lot of hype about our shiny new observatories and the new windows to the universe that they open. There’s the James Webb Space Telescope and its infrared supervision and of course LIGO with its ability to see gravitational waves. And then there’s neutrino astronomy. It gets far less attention in the media - in fact it’s almost as elusive as the particle it depends on - and yet mapping the neutrino sky will surely unlock secrets no other method can access.

Today we’re going to look at some of the first neutrino astrophysics in this new astronomical era, as we discuss a new result from the IceCube collaboration. They report seeing neutrinos produced in the colossal magnetic fields surrounding a black hole with the mass of 10 million Suns. No big deal.

First up, let’s review what neutrinos are. These elementary particles are fermions - so particles of matter rather than force-carrying bosons like the photons of regular astronomy, and their fermion type is lepton, so, cousins of the electron, muon and tau particle. There’s one neutrino type corresponding to each of these three; with the most striking differences being that neutrinos are electrically neutral and have much lower masses than the charged leptons. Their extremely low mass means that neutrinos tend to travel at very close to the speed of light because it doesn’t take much energy to get them close to the cosmic speed limit. But perhaps the most characteristic property of the neutrino is that they only interact via the weak nuclear force and gravity. That makes them very, very difficult to detect. For reference, if you want to stop a low-energy neutrino with a wall of lead, the wall would need to be a light year thick to have even a 50-50 chance that the neutrino gets close enough to a lead nucleus to interact.

So you can see how doing neutrino astronomy might be challenging, given that each neutrino has a miniscule interaction probability most pass straight through your detector. But miniscule isn’t zero. With enough neutrinos you will get interactions. And space does generate a lot of neutrinos. By far the brightest neutrino source on the sky is the Sun. It creates 10^38 - a hundred trillion trillion trillion neutrinos every second in the fusion reactions in its core. These spread out in every direction, so a mere 100 trillion pass through your body every second. In fact, the vast majority pass straight through the entire planet. To have a chance of catching any you need a big detector, and the biggest neutrino detector is IceCube.

There are two famous musicians who are also astrophysicists - those are Brian May and Brian Cox - so no, IceCube is not named after the rapper. It’s named for the fact that it’s literally an ice cube. It’s a full cubic kilometer of glacial ice at the South Pole. Let’s see how ice can be used to see neutrinos.

When neutrinos pass through this region of glacier, 1 in a million interact with water molecules via the weak force. For high-energy neutrinos, interaction is with an atomic nucleus and it can transmute the neutrino into its high-mass lepton counterpart - an electron, muon or tau. If the neutrino becomes a tau then it decays almost instantly, but an electron or muon will continue through the ice, emitting light as it interacts with other charged particles.

This is seen as a cone of blue light that trails the particle. It trails because the lepton is traveling faster than light. See, the speed of light is reduced in any medium. In ice it’s around 25% slower. But neutrinos aren’t slowed down, so the electron or muon that it creates also starts out with a speed faster than the reduced speed of light in the ice. The result is the electromagnetic analog of a sonic boom. The expanding EM waves created by the charged particle expand slower than the particle itself, so their wavefronts overlap each other and you get constructive interference in this cone shape that follows the particle. This is Cherenkov radiation, and it’s ultimately how we detect our neutrinos.

The actual detectors are sensitive photomultipliers - basically extremely sensitive light detectors - suspended in deep boreholes. A grid of over 5000 photomultiplies spans a cubic kilometer of the glacier, starting at a depth of 1.5km. In this way, IceCube sees the Cherenkov radiation from neutrinos generating both electrons and muons, but it’s the muons that are really useful. Electrons interact very strongly with the water molecules and so begin to bounce around immediately, leading to a bubble of Cherenkov near their creation point. But muons can travel for kilometers without changing course, and so their Cherenkov cones trace out a straight line in the direction of the original neutrino path. In this way it’s possible to figure out where on the sky the neutrino came from, and with enough neutrinos even build up a rather blurry image.

We need to sort out one more thing to understand how IceCube actually takes neutrino pics of the sky. It turns out that our environment is very noisy with neutrinos. The sun produces an enormous number, but those are easy to distinguish because they come from the direction of the Sun. More challenging are confounding particles from our own atmosphere. When cosmic rays hit molecules in our atmosphere, many different particles can be produced, but the most annoying are muons and neutrinos. Those muons can cause Cherenkov cones, but they’re easy to sort out because almost all of them come from above. If we only pay attention to signals from below we eliminate atmospheric muons. This makes a neutrino observatory the only type of observatory where you need to wait until your object sets below the horizon before you can observe it.

Atmospheric neutrinos are still a problem, because if they’re produced on the other side of the planet they can still reach IceCube. In order to confidently say you detected the neutrinos of a cosmic object, you need to collect significantly more neutrinos from that particular direction than the smooth background neutrino rate would give you. Which is why it’s taken a decade of collecting neutrinos to confidently detect the amazing space object that we’re finally ready to talk about.

This is the neutrino map of the sky produced by IceCube since it began operations in 2011. And this spot here is the only place where we see a significant over-abundance of neutrinos. There’s only about 50 neutrinos more than would be expected from cosmic rays, but it’s pretty clearly something. There’s only a 1 in 10,000 chance of getting such an over abundance anywhere on the map by pure chance, which in science-speak means a 4.2 sigma detection.

So what could it be? Well, this blurry patch of sky in the constellation of cetus includes a lot of Milky Way stars, a lot of very distant galaxies, but there’s only one thing that is a plausible source of neutrinos. And that’s this thing. This is M77, also known as NGC 1068. It’s a beautiful spiral galaxy 47 million light years away. A regular galaxy should not produce so many neutrinos. But M77 is different. At its heart it holds a terrible secret. As with most galaxies, including our own, there’s a gigantic black hole at its core. In this case with a mass of around 10 million suns. But unlike most galaxies, M77s supermassive black hole is not quiet. It’s currently in a feeding phase - gas from the surrounding galaxy has been driven to the center, forming a whirlpool of searing plasma - an accretion disk - as it falls into the black hole. M77 has what we call an active galactic nucleus. It’s an AGN. The most powerful AGNs are called quasars, and shine out from across the universe. M77 is known as a Seyfert galaxy, and the light of its accretion disk is hidden from us by a wreath of dust and gas surrounding the nucleus.

Being one of the closest AGNs to our galaxy makes M77 a prime candidate as the source of neutrinos. We know that AGNs have powerful magnetic fields because in many we see jets of high energy particles blasted out from the vicinity of the black hole, and these have all the signatures of having been accelerated and focused by magnetic fields. They’re natural particle accelerators, far more powerful than the ones we can build on earth. And collisions of magnetic-field-accelerated particles is exactly how we make neutrinos in our experiments.

On that note, I should mention that M77 may not be the first AGN to be detected by IceCube. In 2017 a single very high energy neutrino was detected coming from the direction of a known blazar. This is an AGN where a magnetically-channeled jet happens to be pointing more or less directly at us. This leads to massive amplification of the jet’s light due to … Einstein stuff. Relativistic boosting due to the jet particles racing towards us at near light speed. Many telescopes swiveled to the blazar after the neutrino detection and it was revealed to be in a particularly active phase, so there’s a good chance it was spitting out high energy neutrinos at an enhanced rate. But there’s only a 3-sigma confidence that the neutrino is associated with the blazar in this case - not enough to claim a sure detection.

That makes the neutrino signal from M77 far more exciting, because we’re approaching certainty in this case. The only other sure neutrino detections are the Sun and the remnant of supernova 1987A in the large magellanic cloud. Which means we can finally start doing real neutrino astrophysics. In fact, that work is well underway. A number of studies already describing the possible mechanisms and environment of the neutrino production. We stand to learn a ton about the regions around these gigantic black holes, and eventually from the many other energetic environments that probably produce neutrinos. Supernovae, colliding neutron stars and black holes, tidal disruption events when black holes rip apart stars, you name it.

To achieve all of this we need bigger and more sensitive neutrino observatories. There are already plans to improve the sensitivity of IceCubes detectors, and also to expand it to a full 10 cubic kilometers, which should approximately increase the detection rate by a factor of 10. We can also look for neutrino Cherenkov radiation at radio wavelengths, which allows us to scan vast tracks of the Antarctic glacier with detectors placed above the ice. In fact we already do so, for example with the ANITA balloon experiment. These experiments can also be expanded. One possibility is looking for radio-Cherenkov from the Moon. So yeah, using the entire moon as a neutrino telescope. The future of neutrino astronomy is bright, and it’s hard to even guess what we’ll learn as we slowly build our neutrino map of space time.