Tag Archives: cosmic rays

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.

The origin of ultra-high-energy cosmic rays

Every so often Earth’s atmosphere gets hit by a charged particle (typically an atomic nucleus) with an energy greater than 1 EeV (1018 eV) — in other words, an ultra-high-energy cosmic ray. One of the longest-standing problems in astronomy is the origin of these ultra-high-energy cosmic rays.

°The reason it’s difficult to pinpoint where these particles come from is that the Milky Way’s tangle of magnetic fields sends electrically charged particles in wild spirals. Any directional information about the origin of these charged particles get destroyed. Cosmic rays thus hit Earth equally from all directions, and that begs the question: do these high-energy particles originate within our galaxy or do they have an extragalactic origin? In a recent paper, astrophysicists at the Pierre Auger Laboratory claim to have solved the mystery: ultra-high-energy cosmic rays have an extragalactic origin.

Surface detector

One of the many water-Cherenkov surface detectors that make up the Auger cosmic ray telescope.

The Auger Collaboration looked at 30000 of the very highest-energy particles. Because of their high energies, these particles undergo less deflection than the billions of low-energy cosmic rays that constantly bombard Earth. The Collaboration tracked these particles back, and found that there was an excess of particles coming from a patch of sky 120° away from the centre of the Milky Way. Furthermore, this patch of sky contains a high density of nearby galaxies.

The case seems to be settled: ultra-high-energy cosmic rays have an extragalactic origin. They come from nearby galaxies. Furthermore, since they don’t come from the Milky Way we can probably assume that the galaxies they do come from are somehow different to the Milky Way. Perhaps they originate in a galaxy such as Centaurus A, which contains relativistic jets powered by a supermassive central black hole. Further study will surely pinpoint the precise origin of these enigmatic particles.

HiSCORE

In New Eyes on the Universe there’s a chapter on cosmic rays, and you can find information there on many of the existing and planned cosmic ray detectors. So you can learn about the Pierre Auger Observatory, the Telescope Array Project, JEM-EUSO and lots of others.

But I missed one out.

The Hundred Square km Cosmic ORigin Explorer (HiSCORE – yes, it’s another acronym) is a Russian-German collaboration aimed at learning more about one of the longest-standing puzzles in astrophysics: the source those mysterious cosmic rays with incredibly high energies. When HiSCORE is completed in 2020 it will have 1000 photomultiplier-based detectors spread over a hundred square kilometres of the Tunka Valley near Lake Baikal, Siberia. The intention is to search for Cerenkov radiation generated when ultra-high energy cosmic rays smash into atoms in the Earth’s atmosphere. The HiSCORE team hopes to study cosmic rays with energies up to an EeV (which is, of course, way beyond anything the LHC can produce).

The first prototypes for the observatory are now being installed, which is why HiSCORE is in the news right now. This burst of interest will fade – but keep an eye on the HiSCORE. Perhaps it will help solve the puzzle of how the hell Nature  packs macroscopic levels of energy into subatomic particles.

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.

Fermi spies a superbubble

One of the mysteries I discuss in New Eyes on the Universe is the origin of ultra-high-energy cosmic rays. These are subatomic monsters, particles that smash into Earth’s atmosphere with macroscopic energy: the famous ‘Oh-my-God’ particle carried 3 x 1020 eV – the kinetic energy of a well-struck tennis ball. What mechanism can accelerate a subatomic particle to that sort of energy? No one knows. However, astronomers are closer to understanding the source of cosmic rays with slightly lower energies (up to about 1015 eV – so still far more energetic than anything the Large Hadron Collider can deliver!)

The Fermi space telescope (previously known as GLAST; as I mention elsewhere, thank heavens that not all astronomy missions are known by acronym) has found evidence for the source of at least some medium-to-high-energy cosmic rays. And thought the details are still to be determined it seems that these cosmic rays are accelerated by shock waves produced when supernovae eject material into space. This model of cosmic ray acceleration, appropriately enough, originated with Enrico Fermi.

Artist's impression of the Fermi space telescope

Artist's impression of the Fermi gamma-ray space telescope.
Credit: NASA

What the Fermi telescope actually found was a source of gamma-rays in the constellation of Cygnus. The source lay along a line between two clusters of stars, the clusters being separated by about 160 light years. One cluster contained over 500 massive stars (the sort of stars that form supernovae), the other cluster contained about 75 massive stars. So how does this relate to cosmic rays?

Well, the clusters contain dense gas clouds – that’s an environment in which massive stars are likely to form – but the stellar wind from a massive star pushes the gas away and creates a ‘bubble’ (When a star explodes as a supernova it also creates a ‘bubble’ around what’s left behind.) These bubbles grow and merge with bubbles around other stars and remnants to form ‘superbubbles’. What Fermi detected (the results are published in Science 334 1103-1107) was high-energy gamma-rays coming from a superbubble in Cygnus. (Since gamma-rays aren’t deflected by magnetic fields, they point straight back to their source; Fermi could thus determine the source of these gamma-rays. Cosmic-rays, being electrically charged, are deflected by the magnetic fields in our Galaxy and around Earth; the arrival direction of a cosmic ray does not necessarily point back to its source.)

The best interpretation of the Fermi data is that cosmic rays were being accelerated by shockwaves in the superbubble; whenever those cosmic rays collided with atoms or molecules inside the superbubble, gamma-rays were produced. The gamma-ray energy distribution was what one would expect from such collisions. Furthermore, the spatial distribution followed the shapes of the gas clouds and cavities. So this is good, strong evidence that some cosmic-rays originate from inside massive-star-forming regions of space.

But what precisely is the acceleration mechanism? An isolated shock wave from a single supernova remnant, or the combined effect of many different shocks? It’s not yet clear. As for the source of the ‘Oh-my-God’ particles – well, God alone knows at present.