Low-Energy Break in the Spectrum of Galactic Cosmic Rays

Measurements of the low-energy spectrum of Galactic cosmic rays (GCRs) by detectors on or near 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 Delta Gamma = 1.1 +/- 0.3 at an energy of E = 9 +/- 3 GeV. Detection of a low-energy break enables a measurement of GCR energy density in the interstellar space U = 0.9 +/- 0.3 eV/cm(3).