Low-energy break in the spectrum of Galactic cosmic rays

TL;DR

This paper uses gamma-ray observations of nearby molecular clouds to measure the Galactic cosmic ray spectrum outside the influence of Solar modulation, revealing a low-energy spectral break at around 9 GeV.

Contribution

It introduces a novel method to determine the GCR spectrum using molecular clouds as natural detectors, bypassing Solar modulation effects.

Findings

GCR spectrum exhibits a low-energy break at about 9 GeV.

The spectral slope hardens by approximately 1.1 at the break.

GCR energy density in interstellar space is measured as 0.9 eV/cm3.

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.