Laboratoire Univers et Théories

Jean Ballet

Particle acceleration in supernova remnants

Évènements associés

  • Le jeudi 2 mars 2017 de 11:00 à 12:00 - lieu : Salle 204, Bâtiment 18, Meudon

The cosmic-rays that pervade the Galaxy (including the Earth’s environment) are thought to be accelerated in strong shocks via a diffusive mechanism (1st order Fermi) originally proposed in 1978. The best place to study that phenomenon is the blast wave created in the interstellar medium by supernova explosions. Theory has it that in young supernova remnants (where the shock velocity is several thousand km/s) the mechanism is so efficient that it transfers a sizable fraction of the kinetic energy to the accelerated particles rather than the thermal gas.
I will present the observational constraints on that mechanism accumulated in X-rays with the Chandra and XMM-Newton satellites. The most spectacular of those is the morphology of the X-ray synchrotron emission in the young supernova remnants such as Cas A or Tycho, concentrated in a very thin shell at the blast wave. The most likely interpretation is that the accelerated particles cool down (due to the synchrotron emission itself) very fast behind the shock. This leads to the idea that the magnetic field was amplified at the shock (by the acceleration process itself) up to several hundred μG. The bilateral shape of SN 1006 illustrates the effect of the orientation of the original magnetic field on this process. I will also describe the search for a precursor to the shock due to the finite diffusion length of cosmic-rays, in X-rays and optical.
Finally, I will discuss the γ-ray emission of supernova remnants, in the light of the results of HESS and Fermi. The GeV range is the best to look for hadronic emission (nuclear interactions between cosmic-ray protons and the interstellar gas, followed by π0 decay) in middle-aged remnants, whereas the TeV range is more sensitive to the leptonic emission (Inverse Compton on CMB, infra-red and optical photons). A few examples such as W 28 allow testing how protons escape from supernova remnants.