Cosmic-Ray Origins

The detection of distributed, high-energy gamma-ray emission points to cosmic-ray acceleration in a superbubble. The origin of cosmic rays has been a mystery since it was conclusively shown by Victor Hess ( 1 ) that ionizing radiation impinges on Earth from space, and subsequently shown by Arthur Compton ( 2 ) that this cosmic radiation is primarily composed of charged particles. Since that time, there has been great interest in understanding the origin of these cosmic nuclei accelerated to nearly the speed of light—identifying the source of the material that is accelerated, the nature of the accelerator, and the mechanism by which the source material is injected into the accelerator. On page 1103 of this issue, Ackermann et al. ( 3 ) report observations with NASA's Fermi Large Area Telescope that are directly related to the origin of cosmic rays. They identified distributed emission of gamma-rays over the energy range of 1 to 100 GeV in the Cygnus X region of the sky with a “cocoon” of freshly accelerated cosmic rays.