Low-energy break in the spectrum of Galactic cosmic rays

Abstract

Measurements of the low energy spectrum of Galactic Cosmic Rays (GCR) by detectors at or near the Earth are affected by Solar modulation. To overcome this difficulty, we consider nearby molecular clouds as GCR detectors outside the Solar system. Using gamma-ray observations of the clouds by the Fermi telescope we derive the spectrum of GCRs in the clouds from the observed gamma-ray emission spectrum. We find that the GCR spectrum has a low energy break with the spectral slope hardening by 1.1+/-0.3 at an energy of 9+/-3 GeV. Detection of a low-energy break enables a measurement of GCR energy density in the interstellar space 0.9+/-0.3 eV/cm3.Comment: 5 pages, 3 figures plus Supplemental Material. Accepted for publication in Phys.Rev.Let