Supernova Remnants - Origin of Cosmic Rays

Origin of Cosmic Rays

Supernova remnants are considered the major source of galactic cosmic rays. The connection between cosmic rays and supernovas was first suggested by Walter Baade and Fritz Zwicky in 1934. Vitaly Ginzburg and Sergei Syrovatskii in 1964 remarked that if the efficiency of cosmic ray acceleration in supernova remnants is about 10 percent, the cosmic ray losses of the Milky Way are compensated. This hypothesis is supported by a specific mechanism called "shock wave acceleration" based on Enrico Fermi's ideas, which is still under development.

Indeed, Enrico Fermi proposed in 1949 a model for the acceleration of cosmic rays through particle collisions with magnetic clouds in the interstellar medium. This process, known as the "Second Order Fermi Mechanism", increases particle energy during head-on collisions, resulting in a steady gain in energy. A later model to produce Fermi Acceleration was generated by a powerful shock front moving through space. Particles that repeatedly cross the front of the shock can gain significant increases in energy. This became known as the "First Order Fermi Mechanism".

Supernova remnants can provide the energetic shock fronts required to generate ultra-high energy cosmic rays. Observation of the SN 1006 remnant in the X-ray has shown synchrotron emission consistent with it being a source of cosmic rays. However, for energies higher than about 1015 eV a different mechanism is required as supernova remnants cannot provide sufficient energy.

It is still unclear whether supernova remnants accelerate cosmic rays up to PeV energies. The future telescope CTA will help to answer this question.