Thermal Processes in Supernova Remnants

This chapter provides a theoretical overview of thermal processes in supernova remnants (SNRs), which can be understood as a series of energy transfers. A part of the kinetic energy associated with expansion of SNRs is converted to the thermal energy of hot plasma via collisionless shock heating that occurs through collective interaction between particles and electromagnetic fields. An important consequence of this process is non-equilibration among the temperatures of different particle species; in the most extreme case, the ratio between the electron and proton temperatures can be as low as their mass ratio at the immediate postshock region. Subsequently, the thermal energy is transferred from the heavier particles (protons and ions) to the lighter ones (electrons) via Coulomb collisions in further downstream regions. Ionization of heavy elements also proceeds through collisions between ions and free electrons. Both processes take several 10,000 years to achieve equilibrium under the typical density of the interstellar medium (ISM). Therefore, non-equilibrium of both temperatures and ionization are commonly expected for young, X-ray bright SNRs. The plasma properties can be investigated through spectral diagnostics of thermal X-ray emission, which is dominated by atomic emission lines, bremsstrahlung, and radiative recombination continua.