A Brief Candle: Cosmic Ray Acceleration in Supernova Remnants
It is generally thought that cosmic rays (CR) are accelerated in supernova shocks. However, while particle acceleration to high energies has now clearly been demonstrated with the current generation of instruments, it is by no means proven that supernovae accelerate the bulk of cosmic rays. The crucial question is whether the emission that is observed comes from electrons – in other words, is leptonic in origin – or from hadrons, which make up most cosmic rays. To determine which it is, we must compare the predictions of acceleration models with observations.
The cosmic ray spectrum observed at Earth can be described by a pure power law up to an energy of a few PeV, where it steepens slightly. The feature is called the “knee”. The absence of other features in the spectrum suggests that, if supernova remnants (SNRs) are the sources of galactic cosmic rays, they must be able to accelerate particles at least up to the knee.
Shockwaves and Magnetic Fields
For cosmic ray acceleration to happen, the acceleration in diffusive shocks has to be fast enough for particles to reach PeV energies before the SNR has swept up much of the interstellar medium (known as the Sedov phase). At this point, the shock slows down and consequently the highest energy CRs can escape. For the required rapid acceleration, strong magnetic fields are needed - 100-1000 times higher than the interstellar medium value.
Such high values of the magnetic fields would rule out a leptonic origin for the observed gamma-ray emission, as the expected synchrotron radiation from relativistic electrons would exceed the measured X-ray emission.
Theory suggests that such an amplification of the magnetic field might be induced by the CRs themselves, and high resolution X-ray observations of SNR shocks seem to support this scenario, though their interpretation is debated. Thus, an accurate determination of the intensity of the magnetic field at the shock is of crucial importance for disentangling the origin of the observed gamma-ray emission and understanding the way diffusive shock acceleration works. For this, we need the excellent angular resolution offered by CTA.
Even if a SNR can be detected by Cherenkov telescopes for a significant fraction of its lifetime (up to several times 10,000 years), it can make PeV CRs only for a much shorter time (several hundred years), due to the rapid escape of PeV particles from the SNR. This suggests that around 10 SNRs currently have a gamma-ray spectrum extending up to hundreds of TeV. The actual number of detectable objects will depend on the distance and on the density of the surrounding interstellar medium. The detection of even a few such objects would be extremely important as it would be clear evidence for the acceleration of CRs up to PeV energies in SNRs. A sensitive scan of the galactic plane with CTA would be ideal to search for these sources.
In addition, the spectra of radiating particles (both electrons and protons), and therefore also the spectra of gamma-ray radiation, should show characteristic curvature (a ‘cut-off’), reflecting particle acceleration at CR-modified shocks. However, to see such curvature, one needs the coverage of a few decades in energy provided by CTA. If the general picture of SNR evolution described above is correct, the position of the cut-off in the gamma-ray spectrum depends on the age of the SNR and on the magnetic field at the shock. A study of the number of objects detected as a function of the cut-off energy will allow tests of this hypothesis and constrain the physical parameters of SNRs, in particular of the magnetic field strength.
Escaping Cosmic Rays
Furthermore, we may be able to observe the diffusion of cosmic rays directly. The presence of a massive molecular cloud near a SNR (or any kind of CR accelerator) provides a thick target for CR hadronic interactions and thus enhances the gamma-ray emission. Gamma-ray studies of molecular clouds can be used to identify the sites where CRs are accelerated.
While travelling from the accelerator to the target, the spectrum of cosmic rays is varies with time, distance to the source, and the (energy-dependent) diffusion coefficient. One may therefore expect varying proton, and therefore gamma-ray spectra. CTA will allow the study of emission depending on these three quantities, which is impossible with current experiments. A determination, with high sensitivity, of spatially-resolved gamma-ray sources related to the same accelerator would lead to the experimental determination of the local diffusion coefficient and/or of the local injection spectrum of cosmic rays.
The observation of cosmic-ray penetration into molecular clouds will also be possible. If the diffusion coefficient inside a cloud is significantly smaller than the average in the neighbourhood, low energy cosmic-rays cannot penetrate deep into the cloud, and part of the gamma-ray emission from the cloud is suppressed. Consequently, its gamma-ray spectrum appears harder than the cosmic-ray spectrum. Both of these effects will be more pronounced in the denser central region of the cloud. With CTA’s angular resolution of the order of 1 arcminute one could resolve the inner part of the clouds and measure the degree of penetration of cosmic rays.
Caprioli et al., The Contibution of Supernova Remnants to the Galactic Cosmic Ray Spectrum, Astroparticle Physics (2010), 33, 3, p. 160-168; http://arxiv.org/abs/0810.4995
Gabici & Aharonian, Searching for Cosmic Ray Pevatrons with Multi-TeV Gamma Rays and Neutrinos, The Astrophysical Journal (2007), 665, 2, p. L131-L134; http://arxiv.org/abs/0705.3011