The Fermi Gamma-ray Space Telescope team reports that they have found a distinctive particle-decay feature in the gamma-ray spectra of supernova remnants IC443 and W44. This finding, reported in Science, validates the theory that the high-energy particles known as cosmic rays are made by the shock waves formed by the supernova events that mark the ends of the lives of high-mass stars. Dr. Charles Dermer, an astrophysicist in the Naval Research Laboratory’s High Energy Space Environment Branch, is part of the Fermi team.
The Fermi team studied the Jellyfish Nebula, IC443, shown here in this ground-based optical telescope view. It is located about 5,000 light-years away. (Photo: Dieter Willasch, Astro-Cabinet)
Dermer explains that Victor Hess pioneered balloon astronomy and made the 1912 discovery of “a radiation of very high penetrating power [that] enters our atmosphere from above.” In 1934, Baade and Zwicky conceived 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. Fermi himself conceived of the idea of statistical acceleration by magnetic clouds in 1949.
However, hard-and-fast evidence for the acceleration of cosmic-ray protons and ions in supernovae, or in anything else for that matter, has been lacking, because tangled interstellar magnetic fields deflect the cosmic rays from the directions to their sources, Dermer explains. But any cosmic-ray factory must be illuminated by the radiations formed when cosmic rays collide with target gas and dust nuclei. Through intermediate neutral pion (pi-zero, π0) production, which sets in when the cosmic-ray kinetic energy is far greater than the π0 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 makes it hard to doubt that dying massive stars manufacture energetic cosmic rays that make the gamma-ray spectrum observed.
For Dermer, working as part of the Fermi team and in his own individual studies, “this discovery represents the culmination of a nearly 30-year avenue of cosmic-ray research,” says Dr. Jill Dahlburg, Superintendent of NRL’s Space Science Division. This kind of high-energy radiation research to investigate the cosmic rays broadly applies to the space radiation environment and provides the Navy with critical knowledge for operational space situational awareness.
(Left) The Fermi Large Area Telescope (LAT) gamma-ray sky map showing the supernova remnant IC 443 in the direction of the Geminga and Crab pulsars. (Right) The gamma-ray spectrum of IC 443, measured by the Fermi Space Telescope between 100 MeV and 100 GeV, and by ground-based gamma-ray arrays in the TeV range. The low-energy cutoff in the spectrum is the pion bump feature (M. Ackermann et al., Science, 339, 807 (2013)).