Cosmic Rays and Supernova Remnants

Mystery of Cosmic Rays

On August 7, 1912, a physicist Victor Hess conducted a balloon flight experiment to investigate natural radiation levels along altitude. He found that the radiation strength became much higher with increasing height against his expectation (Hess, 1912). The radiation event-rate was doubled at an altitude of 5 km. This discovery was confirmed by Robert Millikan in 1925, and he gave the name “cosmic rays” (CRs) to the radiation (Millikan, 1925). The composition of CRs was disentangled from the 1920s to the 1940s. About 90% of CRs is protons, and most of the rest is He. The origin of the CRs was discussed by Enrico Fermi in 1949 (Fermi, 1949). Basic idea is that charged particles are reflected by interstellar magnetic mirrors which are moving forward and backward, and gradually gain energy on average. This random acceleration mechanism is called “second order Fermi acceleration”. After the 1960s, the spectral energy distribution of CRs were started to be investigated using particle detectors on satellites. Now, the energy spectrum is known to have power- law shapes with spectral indices1 of 2.7 below 1015.5 eV (“knee” energy), 3.0 between 1015.5–1018.5 eV (“ankle” energy), and 2.6 above “ankle”. Because of these spectral shapes, two categories of CRs with separate origins are believed to be mixed: “Galactic cosmic rays” and “extragalactic cosmic rays”. Galactic and extragalactic CRs are thought to dominate the energies below and above the ankle, respectively (e.g., Hillas 1984). The energy density of CRs has been found to be ∼ 1 eV cm−3 (Gloeckler & Jokipii, 1967). The chemical composition of CRs has large excess from the solar system abundances especially of light elements and Fe-group elements (Simpson, 1983). The most likely candidate of the origin of the Galactic CRs is supernova remnants (SNRs), because they are almost only sources that can supply the total amount of energy of the Galactic CRs. In addition, since progenitor stars of certain type of SNRs (remnants of Ia-type supernovae) include large amount of metals including Fe-group elements, SNRs may be able to explain the abundance of CRs. Concerning the acceleration mechanism, shock waves of SNRs are thought to cause an efficient acceleration process called “first order Fermi acceleration”. This acceleration mechanism was proposed in 1977–1978 by several researchers and are thought to be the most plausible mechanism for production of Galactic CRs (Axford et al., 1977; Krymsky, 1977; Blandford & Ostriker, 1978; Bell, 1978).

  • How are Galactic / extragalactic cosmic rays accelerated ?
  • Where are they accelerated ?
  • How much amount of energy is accelerated ?
  • When and how are they released into the universe ?

Supernova Remnants as Cosmic-Ray Accelerators

There are two principal scenarios for the cause of supernova explosions: core collapse and thermonuclear supernovae. Core collapse supernovae are caused by the end of the lives of massive stars: main sequence stars with masses M ≳ 8M⊙ (e.g., Woosley & Janka 2005). Thermonuclear supernovae originate from explosive nuclear burnings of white dwarfs when their masses become larger than the Chandrasekhar limit (≈ 1.38M⊙: Chandrasekhar 1931; Hawking & Israel 1989) due to mass accretion from their companion stars. The ejecta of supernova explosions travels with high velocities, which are typically ESN,kin/2Mej ∼ 104 km s−1, where ESN,kin is the kinetic energy of a supernova explosion and Mej is the ejecta mass. These are larger than the sound velocities in the ISM, which are ∼ 10 km s−1, where γad is the adiabatic index, so that they produce shocks in the ISM. Along with the particle acceleration processes, the shock sweeps up the ISM into a hot plasma, which mainly emits X-rays. Thus, the physical parameters in the vicinity of the shock, which are crucial to the acceleration processes, can be measured mainly by X-ray observations.

  • What is the maximum achievable energy of acceleration in supernova remnants ?
  • What is the energy distribution of accelerated particles diffusing into the universe like ?
  • In which environments do accelerated particles diffuse from the acceleration sites ?
  • Does shock-cloud interaction increase or decrease magnetic field turbulence ?

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