Recent gamma-ray observations of Tycho’s supernova remnant (SN 1572) by the space-based Fermi-Large Area Telescope (Fermi-LAT) and the Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Southern Arizona, have provided new fundamental pieces of information for understanding particle acceleration and non-thermal emission in supernova remnants.
SN 1572 was a Type Ia supernova in the constellation Cassiopeia and is one of about eight supernovae visible to the naked eye in historical records. It was discovered in November 1572 by a number of different individuals. The astrophysicists developed a coherent description of Tycho’s properties in terms of supernova remnant evolution, shock hydrodynamics and multi-wavelength emission by accounting for particle acceleration.
The astronomers discovered that Tycho’s forward shock accelerates protons to at least 500 TeV, channeling them into cosmic rays, at about 10% of its kinetic energy. The cosmic-ray-induced streaming instability is consistent with all of the observational evidence of very efficient magnetic field amplification. In such a strong magnetic field, the velocity of the Alfvén waves, a type of magnetohydrodynamic wave that travels at low-frequency through plasma and is dispersionless, scattering the cosmic rays in the upstream is expected to be enhanced.
The accelerated particles feel an effective compression, in turn leading to an energy spectrum steeper than the standard prediction. In such a hadronic scenario, along with the inability of the concurrent leptonic mechanism, which inverses the Compton scattering of electrons on several photon backgrounds, to reproduce both the shape and normalization of the detected gamma-ray emission. This represents the first directive radiative evidence that hadron acceleration occurs efficiently in young galactic supernova remnants.