Galactic Cosmic Rays and Supernova Remnants

Cosmic rays that are permanently detected on Earth come from sources in our Galaxy, as long as energies below 10¹⁵-10¹⁸ eV are concerned. This is derived from the fact that the Sun and, by inference, solar-like stars only sporadically accelerate particles to energies that allow their detection on the Earth, while cosmic rays at higher energies than the above limits would not be confined in our Galaxy. The measured abundances of different elements in the cosmic ray population detected on Earth also argue in favour of a galactic origin. The possible acceleration regions are supernovae and their remnants, which drive powerful shocks.

It is not easy to identify the regions where cosmic rays come from, because charged particles do not propagate along straight lines. We localise stars because their light comes from a given direction in the sky. We know that cosmic rays at energies up to 10¹⁰ eV (10 GeV) may occasionally come from the Sun, travelling along interplanetary magnetic field lines. But the continuous flux of cosmic rays up to 10¹⁹ eV comes to the Earth from any direction in the sky. This is because the charged particles travel along turbulent magnetic field lines in our Galaxy. Since the magnetic field guides the particles, the continuously changing direction of a turbulent field continuously changes the direction of the particle motion. As a result, cosmic rays are scattered, like molecules in a hot gas, and their original travel direction is completely washed out at particle energies relevant to neutron monitor measurements.

Given the modest energies of the most energetic particles from the Sun as compared to more than 10²⁰ eV in the most energetic cosmic rays, it is clear that most cosmic rays cannot be accelerated in normal stars like the Sun. Some special conditions, events where large amounts of energies are released, must be at the origin of high energy cosmic rays.

On this page we limit ourselves to the energies that can be observed with neutron monitors on the ground, and discuss galactic cosmic rays. Galactic cosmic rays can have energies far beyond those observable with neutron monitors. It is presently thought that protons up to 10¹⁵ eV come from our Galaxy, as well as ions at up to a thousand times higher energies.

Cosmic rays at still higher energies are an extremely interesting subject that is presently addressed with powerful new instruments. More information can be found, for instance, on the web sites of the Auger and TAL telescope cooperations.

Where are cosmic rays accelerated?

As already said, solar particle acceleration to energies as high as 10 GeV has been observed, but it is sporadic. The impact of cosmic rays on the Earth, however, is permanent. So most cosmic rays cannot come from the Sun or solar-like stars.

Clues from elemental abundances

Another reason to believe that most cosmic rays come to us from some distance is suggested by the abundances of the different chemical elements in cosmic rays.

This figure (dead link) compares the abundances of cosmic rays measured with satellites near the Earth (blue line) with the average abundances of elements in the solar system (red bars). The horizontal axis gives the number of protons in the nucleus, and the symbol of the corresponding chemical element is noted at the top of the diagram. The abundances are expressed with respect to that of Si (abundance 100 in this plot): for one nucleus of Si (14 protons) there are more than ten thousand nuclei of H (1 proton) and 1 of Fe (26 protons).

For most elements the relative abundances in cosmic rays and the average solar system abundances are similar. This does not mean that the cosmic rays come from the solar system, because the solar system abundances themselves are similar to the overall elemental abundances in our Galaxy and others. But there are also differences: the light nuclei hydrogen (H) and helium (He) are less abundant in cosmic rays than in the solar system, which may be a consequence of the acceleration process. Two groups of elements are relatively much more abundant in cosmic rays than in the average Universe: the light elements lithium (Li), beryllium (Be) and boron (B), which comprise 3 to 5 protons, and the heavy elements with 21 to 25 protons (scandium Sc, titanium Ti, vanadium V, chromium Cr, manganese Mn).

Why are these particle species so much more abundant in cosmic rays than in the average Universe? Note that for both groups there are abundant elements which are slightly heavier: C, N, O for the group of light elements, Fe and others for the group of heavy elements. This suggests an explanation for the overabundance in the cosmic rays: the bulk of the cosmic ray Li-B and Sc-Mn nuclei were not part of the initially accelerated population, but were created by collisions of the originally accelerated particles with ambient nuclei in interstellar space. The collisions destroyed the heavier nuclei and created energetic debris - the overabundant cosmic ray species. This interpretation requires in turn that the cosmic rays must have traversed a minimum amount of matter on their way from their source to us, and we can deduce their age and travel distance: the travelled distance is not the same for every cosmic ray species, but on average it turns out to be greater than our Galaxy. Given the indication that cosmic rays undergo a complex trajectory in turbulent galactic magnetic fields, this result is consistent with an origin of cosmic ray protons up to 10¹⁵ eV and ions up to perhaps 10¹⁸ eV in our Galaxy.

Supernovae and shock waves

We therefore have to search for violent events in our Galaxy to identify the origin of cosmic rays. An extreme case of energy release in our Galaxy is a supernova: the collapse of a massive star at the end of its life, when it has no more means to maintain its equilibrium and energy generation through nuclear fusion in its interior. When the core of such a star implodes, its outer layers, containing several solar masses of gas, are expelled at huge speeds into interstellar space. Like a supersonic airplane in the Earth's atmosphere this violent movement of matter generates a shock wave. In an ionised gas shock waves are thought to be efficient accelerators of charged particles. Supernova shocks have initial speeds of several thousands of km/s. They decelerate over tens of thousands of years.

We can see today the remnants of supernovae that exploded in the more or less distant past. An example is the remnant of a supernova in the year 1006, which at that time was very bright for a few weeks and appeared to the observers as a “new” star. At the place of this star we see today a nearly spherical nebula, represented in the Figure above by two grey-scale images (reverse colour scale: dark shading shows bright emission). The observations were made at radio frequencies (843 MHz) and X-rays (see dead link).

The supernova remnant appears with a shell structure that shows heated gas (X rays) and high energy electrons (radio waves and X-rays) around the shock wave that propagates into the ambient interstellar medium. The colour picture on the right shows a combination of more recent maps of the object, combining X-ray observations from the Chandra telescope (blue) with different optical images (yellow, orange, light blue) and a radio image (red). Source.

What do these images tell us about energetic charged particles? From observations at different frequencies we know that the radio emission and part of the X-rays are synchrotron radiation. Synchrotron radiation comes from very high energy electrons or positrons circulating around magnetic field lines. The frequency of this radiation is the higher, the higher the energy of the particle. From our knowledge of the synchrotron mechanism and interstellar magnetic fields we deduce that the electrons emitting radio waves at 843 MHz have energies of a few GeV (10⁹ eV). X-rays are electromagnetic waves at much higher frequencies than radio emission. So the energies of the emitting electrons or positrons must be much higher than those emitting the radio waves. The X-ray emission reveals electrons with energies as high as 10¹⁴ eV.

So supernova remnants are clearly sources of high energy electrons in the cosmic ray population. But what about protons and nuclei? Unfortunately we have much less indications from electromagnetic radiation for them. Protons and nuclei emit some radiation through nuclear interactions. The clearest signature are gamma rays from decaying neutral pions. Neutral pions are unstable particles. They are produced when a high energy proton hits a proton or nucleus of the interstellar medium. The neutral pion decays nearly immediately into gamma rays with a range of energies around 67 MeV, when measured in the frame where the pion is at rest. If the decaying pion moves at large speed, the emission can be seen at much higher photon energies, extending to the TeV (10¹² eV) range.

But this emission, if it exists, is hidden within different types of radiation of energetic electrons. The HESS telescope array in Namibia has seen extended emission at very high energy gamma-rays (above 100 GeV), which is attributed to cosmic ray protons and nuclei. The identification of the gamma-ray spectrum from pion decay is also a key project for the new FERMI telescope.

The acceleration of charged particles at shock waves

A shock wave acts on a charged particle like a tennis racket acts on the ball: if the player hits the ball with a rapid forward move of the racket, the ball will be reflected with a high speed, higher than the speed at which it came in. The ball has been accelerated.

At a shock wave in an ionised and magnetised gas, the particle is reflected because the magnetic field is compressed and intensified behind the shock. The reflection of the particle is similar to the reflection at the Earth's magnetosphere, which prevents low-energy cosmic rays from entering into the atmosphere. In the case of the magnetosphere, the charged particle encounters an object that is practically at rest. The particle is reflected with the same speed with which it impinged on the magnetosphere - just as if the tennis player did not move the racket when it is hit by the ball. But the shock wave is not static. It travels away from the parent star, into interstellar space. After reflection by this travelling shock the particle has a higher speed than before - the charged particle has been accelerated by the encounter with the supernova shock. A single encounter does not increase the energy very much, but when the particle encounters many shocks or many times the same shock, it can be accelerated to considerable energies. This is the present idea of how protons and nuclei are accelerated up to energies of about 10¹⁵-10¹⁸ eV, the “knee” of the cosmic ray spectrum.

The limiting energy to which particles can be accelerated depends on the available time and on the capacity of the medium to reflect energetic particles back to the shock so that they get a supplementary kick. Supernova shocks evolve: they are vigorous for some time, but gradually lose power as they propagate into the ambient interstellar medium and accelerate particles. This is why researchers believe that cosmic ray protons at energies well above 10¹⁵ eV and ions above 10¹⁸ eV need a more powerful accelerator that is not available in our Galaxy.