Tag Archives: IceCube

The oldest problem in astronomy – solved?

It’s probably the oldest problem in astronomy: what’s the origin of high-energy cosmic rays? Finally, the question might have been solved.

Victor Hess discovered cosmic rays back in 1912, but it proved incredibly difficult to identify the astrophysical source of these bullets. The obstacle to progress was the fact that charged cosmic rays – whether protons or atomic nuclei – don’t follow a straight-line path from source to Earth. Instead, the paths get bent and twisted by magnetic fields in space. Just because a cosmic ray appears to come from a particular direction of sky doesn’t mean it really did come from that direction. It seems to be an insurmountable problem.

But we are now in the era of multi-messenger astronomy! And that allows astronomers to answer questions that once seemed impossible.

The key to unlocking the cosmic ray mystery is that the violent events that generate high-energy charged particles will also generate neutrinos. And neutrinos do follow a straight-line path from source to Earth: because they interact solely via the weak force their paths aren’t bent by magnetic fields, and they don’t get absorbed or scattered by intervening matter. Neutrinos can act as tracers of high-energy cosmic rays. Of course, the same properties that make them useful tracers also make them incredibly difficult to detect: indeed until recently, apart from a diffuse neutrino background,  astronomers had managed to confirm only two astrophysical sources of neutrinos: the Sun and SN1987A (the latter being a relatively close supernova). The IceCube observatory, however, now has good evidence for a third source: TXS 0506+056. And this might have solved the mystery of high-energy cosmic rays.

In September 2017, IceCube – a neutrino telescope consisting of detectors buried in a cubic kilometer of South Pole ice – spotted a neutrino with an energy of 290TeV. (That’s 40 times more energetic than the particles accelerated by the LHC.) Astronomers could trace it back to a source in the direction of Orion. IceCube sent out an alert to observatories around the world, and several of them – Fermi, MAGIC, HAWC and others – detected an increase in gamma-ray activity from the same patch of sky. The culprit was TXS 0506+056 – a blazar that’s about four billion light years away.

Artist's depiction of a blazar

A blazar is an active galactic nucleus in which one of the jets points directly at Earth. Charged particles are deflected by magnetic fields, but neutrinos and EM radiation can head straight towards Earth. Needless to say, this artist’s depiction is not to scale! (Credit: IceCube/NASA)

A blazar is an active galactic nucleus – the compact central region of a galaxy where a supermassive black hole sucks material onto an accretion disk and spews out radiation in two opposing relativistic jets. When we see a blazar, we just happen to be looking directly down one of the jets. It’s quite a thought: four billion years ago the central black hole of a galaxy hurled neutrinos and charged particles and gamma radiation towards Earth. Magnetic fields steered the charged particles away from us. But the neutrinos and gamma rays made it to Earth. And, in September 2017, IceCube detected one of those neutrinos.

The “Case of the High-Energy Cosmic Rays” isn’t entirely closed. Astronomers would want to see more examples before they can be sure that active galactic nuclei are the source. But the observation is very, very suggestive.

And, as with all else in science, the answer to one question raises others: Can other objects besides active galactic nuclei produce high-energy cosmic rays? What is the exact mechanism whereby these particles are produced? And what is the source of the most powerful cosmic rays – are blazars responsible for them too? Now that we are in the age of multi-messenger astronomy, an age in which we can observe astrophysical events not only across the entire electromagnetic spectrum but also with gravitational wave telescopes and neutrino telescopes … well, the answers might start to come more quickly.

Bert and Ernie and a new type of astronomy?

In May 2013, scientists presented a preliminary analysis of 28 high-energy events captured by the IceCube Neutrino Observatory, a strange telescope entombed deep in Antarctic ice. Two of these events – dubbed Bert and Ernie – had an energy above 1 PeV. (I wrote about these events in an earlier post.) The other 26 events had an energy in excess of 30 TeV. The initial analysis suggested that these 28 events were likely to be from extraterrestrial sources. A more detailed analysis, published today in the journal Science, suggests that only about 11 of the 28 events are likely to have been caused by atmospheric muons or neutrinos. This means that, at a 4? level of certainty, IceCube has detected high-energy neutrinos from outside the Solar System. A 4? result is not quite at the 5? level that is usually said to constitute a discovery, but it is highly suggestive: there is only one chance in 15000 that all those detections were of purely atmospheric events.

IceCube building

The IceCube Neutrino Observatory consists of dozens of photomultiplier tubes attached to 86 cables, each of which are up to 2.5 km long and buried deep in Antarctic ice. The photomultipliers detect the Cerenkov radiation from fast-moving secondary particles created when neutrinos strike nuclei in the ice. The structure here is just the tip of the observatory! (Credit: IceCube Collaboration)

The exciting thing, I believe, is that the IceCube team now know how and where to look for high-energy neutrinos. They’ll find more astrophysical neutrinos, for sure, and the neutrino sky suddenly looks much more interesting. For many years, the only extraterrestrial neutrinos that astronomers had detected were those from the Sun and a few from SN1987A. IceCube has thus broken new ground.

The IceCube discovery has caused many commentators to hail a new type of astronomy: neutrino astronomy. Well, I don’t think we are quite there yet. The problem is that we don’t know where Bert, Ernie or the other neutrinos originated. To do neutrino astronomy one needs to be able to correlate neutrinos with specific astrophysical objects; the IceCube measurements lacked the angular resolution to do this. But that, too, will come. And new neutrino telescopes, such as the KM3NeT facility that is being constructed in the Mediterranean, will help.

We can’t do neutrino astronomy just yet, but it won’t be long before we’re studying the universe from an entirely new vantage point. And then, for the first time, astronomers will be able to study the distant universe using something other than electromagnetic radiation. IceCube is opening its eyes.

Bert and Ernie – dark matter candidates?

At the time New Eyes on the Universe was published, the only confirmed sources extraterrestrial neutrinos were the Sun and SN1987A. The view of the sky afforded by neutrino telescopes was rather dull.

That view of the neutrino sky is beginning to change. The IceCube SouthPole Neutrino Observatory – a “telescope” consisting of particle detectors buried in one cubic kilometre of Antarctic ice – has detected 28 neutrinos with an energy in excess of 30 TeV (a teraelectronvolt is 1012 eV). Two of these neutrinos, dubbed Bert and Ernie, had energies in excess of 1 PeV (that’s 1015 eV) – far in excess of energies available at the Large Hadron Collider.

Artist's impression

An artist’s impression of the array of optical sensors, buried in Antarctic ice, that form the IceCube telescope. If a high-energy neutrino interacts with an oxygen atom in the ice, a charged particle can be produced that will be moving through the ice faster than light itself can travel through the ice. A cone of Cerenkov radiation, with its characteristic blue hue, will be produced – and it’s this radiation that the sensors detect. (Credit: IceCube Collaboration/NSF)

It’s possible that Bert and Ernie were produced by high-energy cosmic rays smashing into Earth’s atmosphere, but an extraterrestrial origin for these neutrinos does seem more likely than not. And If IceCube has indeed detected high-energy neutrinos from the depths of space the question becomes: what was their source? That’s where things get interesting. If they came from some violent astrophysical source then astronomers have a telescope that lets us study them. Or perhaps they came from the decay of dark matter particles – a suggestion made in a recent preprint by Arman Esmaili and Pasquale Serpico (Are IceCube neutrinos unveiling PeV-scale decaying dark matter?). Whatever the source of Bert and Ernie turns out to be, it seems certain that IceCube truly is giving us some new eyes through which to view the universe.

The cosmic ray gun

It’s one of the most long-lasting questions in astrophysics: what’s the source of those really high-energy cosmic rays that sometimes hit Earth? What cosmic gun could possibly shoot such high-energy bullets towards us?

There are two obvious candidates: active galactic nuclei and gamma ray bursts.

There are seem to be two types of progenitors for gamma ray bursts, but the most luminous events probably come from the collapse of very massive, rapidly rotating stars. Models of such collapse events suggest that the fireball should, alongside the generation of extremely energetic gamma rays, generate high-energy cosmic rays and neutrinos. And scientists now have a detector that can hunt for neutrinos from gamma ray bursts: IceCube.

Artist's impression of the IceCube observatory

An artist's depiction of the IceCube observatory: 86 strings containing 5160 Digital Optical Modules are used to detect neutrino events.
(Danielle Vevea/NSF & Jamie Yang/NSF)

I don’t propose to discuss IceCube in detail in this post. I’ll surely do that in later posts, and you could always read the relevant chapter in New Eyes on the Universe. The exciting news yesterday is that IceCube has been used to look for neutrinos from 300 gamma ray bursts detected by the Swift and Fermi space telescopes. Neutrinos are of course notoriously difficult to spot, but IceCube should have seen several neutrinos from these exceptionally luminous events. It saw nothing.

This negative result suggests that our models of particle production in the fireball of a gamma ray burst might need some major tweaking – and perhaps that high-energy cosmic rays don’t come from burst after all. Perhaps active galactic nuclei are the guns that fire those cosmic ray bullets at us.