Sources of Galactic Cosmic Rays

Sources of galactic cosmic rays

Supernova Remnants (SNRs)

Supernova remnants (SNR) remain the most likely candidates for CR acceleration up to at least $10^{14}$ eV via the Fermi shock mechanism. Supernova explosions are very violent events which transfer a significant amount of energy in the ISM. The explosion mechanism can be the carbon deflagration of white dwarts (Type I) or the core collapse of masssive stars (Type II) but the dynamical evolution of the supernova remnant (SNR) i.e., the expanding cloud of hot gas in the ISM is similar:

• Free Expansion Phase. The shock wave moves in the ISM gas a highly supersonic speed. The shock radius scales as: . Behind the shockfront ISM gas starts to accumulate an a reverse shock starts to form. Sometimes we see first this reverse shock. At some point the compressed ISM gas equals the ejected material, this marks the end of the free expansion phase. It last less than 200-300 years.

• Sedov-Taylor Phase. Once the reverse shock reaches the nucleus, the interior of the SNR gets very hot that energy losses due to radiation are not possible (all atoms are ionized). The cooling of the gas is only due to the expansion, that's why this phase is the adiabatic phase. The radius goes as . When temperature reaches the critical value of $10^6 °K$ ionized atoms start to capture free electrons and can lose energy due to de-exitation. This is the end of the adiabatic phase. This phase can last 20'000 years. This is the phase when Cosmic Rays are mostly accelerated.

• Cooling or Snowplough phase Due to the effective radiative cooling the thermal presure decreases and the expansion slows down. More and more interstellar gas is accumulated until the swept-up mass is much larger than the ejected material. Finally the shell breaks up into clumps probably due to Rayleigh-Taylor inestabilities. This phase lasts up to 500,000 years.

The Tycho SNR

Source:X-ray: NASA/CXC/Rutgers/K. Eriksen et al.; Optical (starry background): DSS

This is a photograph of the Tycho supernova remnant taken by the Chandra X-ray Observatory. Low-energy X-rays (red) in the image show expanding debris from the supernova explosion and high energy X-rays (blue) show the blast wave, a shell of extremely energetic electrons. The X-ray emission of the debris is due to the reverse shock wave racing inward at Mach 1000 which is heating the remnant and causing it to emit X-ray light.

Other sources of Galactic Cosmic Rays

• Neutron stars Neutron stars, especially young fast-rotating pulsars and magnetars have extreme magnetic fields (up to $10^{12}$ G in the case of magnetars) with complex structure that could accelerate CR up to the highest energies. These objects are far rarer than SNRs, however, only a dozen magnetars are known in the Milky Way, although many could exist in the local neighborhood.

• Microquasars are radio-intense X-ray binary stars with companion orbiting an accreting black hole. They are particularly interesting particle accelerators due to observation of VHE gamma ray emission and highly relativistic jets which could provide energy for UHECR