Fermi-LAT g-ray spectrum of diffuse Galactic gas (Abdo et al. Astrophys. J. 703, 1249, 2009) fit with a model where the gas is bombarded by cosmic rays to make secondary pion decay g rays. The particle spectrum is a momentum power law with index s. Different nuclear production models and metallicity correction factors k are considered
Fermi-LAT g-ray spectrum of diffuse Galactic gas (Abdo et al. Astrophys. J. 703, 1249, 2009) fit with a model where the gas is bombarded by cosmic rays to make secondary pion decay g rays. The particle spectrum is a momentum power law with index s. Different nuclear production models and metallicity correction factors k are considered

Objective
Understand the sources of cosmic rays, the extraterrestrial radiation discovered in Victor Hess’ pioneering balloon experiment, in which he discovered “a radiation of very high penetrating power [that] enters our atmosphere from above.” The idea of an origin of cosmic rays from supernovae—cataclysmic explosions taking place when a high-mass star exhausts its fuel, after which its iron core collapses to form a neutron star and a core-bounce-driven explosion—was made by Baade and Zwicky in 1934. Fermi himself conceived of the idea of statistical acceleration by magnetic clouds in 1949. Hard-and-fast evidence for the acceleration of cosmic-ray protons and ions in supernovae has been lacking because tangled interstellar magnetic fields misdirect the cosmic rays from their sources. But any cosmic-ray factory must be illuminated by the radiations formed when cosmic rays collide with target gas and dust nuclei.

Approach
Fermi Large Area Telescope (LAT):

  • NRL is one of a consortium of US and international partners for the Fermi Gamma-ray Space Telescope (NASA/DoE), and led development of the Fermi-LAT calorimeter.
  • Fermi-LAT data on diffuse Galactic gas is analyzed and fit

Nuclear Production Physics and Model Comparisons

  • Compare hybrid isobaric/scaling model of Dermer, Astron. Astrophys. 157, 223 (1986) with model of Kamae et al. Astrophys. J. 647, 692 (2006) using parametric representations of, e.g., diffractive and scaling processes
  • Test cosmic-ray spectra described by power laws in momenta, as predicted by shock-acceleration theory

Deliverable/Value/Accomplishment

  • Through intermediate neutral pion (p0) production, which sets in when the cosmic-ray kinetic energy is far greater than the p0 rest mass energy of 135 MeV, gamma-rays are made with a significant deficit of energy on the low-energy side of the spectrum. The pion bump found in Fermi Space Telescope data from two supernova remnants IC443 and W44, and reported in the journal Science, makes it hard to doubt that dying stars engineer the cosmic rays that make the observed gamma ray spectrum (Ref: M. Ackermann et al., Science, 339, 807 (2013))
  • This discovery validates the theory that the high-energy particles radiations known as cosmic rays are made by the shock waves formed by the supernova events marking the end of a high-mass star