Seminar: John Hewitt
Nature's Accelerators and the Mysterious Origin of Galactic
The origin of cosmic rays in our Galaxy is a century-long puzzle. Charged particles with energies up to a few PeV permeate interstellar space. It has long been thought that these cosmic rays are accelerated in the expanding shockwaves of supernovae. Observations with modern gamma-ray telescopes can be used to trace cosmic rays in interstellar space interacting with the ambient gas and low-energy radiation fields, turning our suspicion into certainty.
NASA's Fermi Gamma-ray Space Telescope has detected a variety of both expected and unexpected Galactic GeV gamma-ray sources, including supernova remnants, pulsars and their nebulae, and stellar novae. Importantly, in the two brightest supernova remnants we detect a low-energy pion-decay cutoff, a definitive signature of cosmic ray protons, finally proving that supernova remnants do accelerate Galactic cosmic rays. The growing number of identified supernova remnants by Fermi allows a comparative study of the effects of evolution and environment on acceleration efficiency. These new results promise to resolve the question of origin, and deepen our understanding of the physics of cosmic ray acceleration and transport in the interstellar medium.