CRA Seminar: Prof. Justin Vandenbroucke (UW Madison)

Cosmic-ray anisotropy searches with the Fermi Large Area Telescope

Cosmic rays arrive at Earth nearly isotropically due to magnetic deflection during propagation. However, small anisotropy has now been measured in cosmic-ray nuclei over eight orders of magnitude in energy, from 100 GeV to 10^10 GeV. This anisotropy is an essential observable in the study of cosmic ray origins and propagation. The Fermi Large Area Telescope (LAT), although primarily a gamma-ray instrument, is making unique measurements at the low end of this range. With a large acceptance and a decade of flight, it has recorded the largest set of cosmic rays ever collected at the 100 GeV energy scale. The LAT has set stringent limits on cosmic-ray electron and positron anisotropy, with implications for dark matter and the mysterious positron excess. Moreover for cosmic-ray nuclei, it has excellent ability to identify composition and to constrain the full-sky anisotropy, neither of which has been possible by the ground-based instruments that have performed all anisotropy measurements to date.