Tag Archives: cosmic rays

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.